Regression: Designer crashes when closing the form

                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                               
1  raise                                                                                                                                                                                                                                        0x7ffff47f79d5 
2  abort                                                                                                                                                                                                                                        0x7ffff47e08a4 
3  qt_message_fatal                                                                                                                                                                                      qlogging.cpp                      1892 0x7ffff4db2361 
4  QMessageLogger::fatal                                                                                                                                                                                 qlogging.cpp                      890  0x7ffff4dae624 
5  qt_assert                                                                                                                                                                                             qglobal.cpp                       3119 0x7ffff4da4314 
6  qt_qFindChild_helper                                                                                                                                                                                  qobject.cpp                       2006 0x7ffff51c5072 
7  QObject::findChild<qdesigner_internal::SignalSlotEditor *>                                                                                                                                            qobject.h                         164  0x7ffff77fd62f 
8  qdesigner_internal::SignalSlotEditorWindow::setActiveFormWindow                                                                                                                                       signalsloteditorwindow.cpp        746  0x7ffff77fb6e7 
9  QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<QDesignerFormWindowInterface *>, void, void (qdesigner_internal::SignalSlotEditorWindow:: *)(QDesignerFormWindowInterface *)>::call qobjectdefs_impl.h                152  0x7ffff77fec4b 
10 QtPrivate::FunctionPointer<void (qdesigner_internal::SignalSlotEditorWindow:: *)(QDesignerFormWindowInterface *)>::call<QtPrivate::List<QDesignerFormWindowInterface *>, void>                        qobjectdefs_impl.h                185  0x7ffff77fe8be 
11 QtPrivate::QSlotObject<void (qdesigner_internal::SignalSlotEditorWindow:: *)(QDesignerFormWindowInterface *), QtPrivate::List<QDesignerFormWindowInterface *>, void>::impl                            qobjectdefs_impl.h                395  0x7ffff77fe44b 
12 QtPrivate::QSlotObjectBase::call                                                                                                                                                                      qobjectdefs_impl.h                375  0x7ffff4e984ff 
13 doActivate<false>                                                                                                                                                                                     qobject.cpp                       3825 0x7ffff51d1142 
14 QMetaObject::activate                                                                                                                                                                                 qobject.cpp                       3885 0x7ffff51caf21 
15 QDesignerFormWindowManagerInterface::activeFormWindowChanged                                                                                                                                          moc_abstractformwindowmanager.cpp 333  0x7ffff6ecc3c7 
16 qdesigner_internal::FormWindowManager::setActiveFormWindow                                                                                                                                            formwindowmanager.cpp             329  0x7ffff76a058e 
17 qdesigner_internal::FormWindowManager::removeFormWindow                                                                                                                                               formwindowmanager.cpp             297  0x7ffff76a0441 
18 qdesigner_internal::FormWindow::~FormWindow                                                                                                                                                           formwindow.cpp                    323  0x7ffff767acef 
19 qdesigner_internal::FormWindow::~FormWindow                                                                                                                                                           formwindow.cpp                    339  0x7ffff767afd4 
20 QObjectPrivate::deleteChildren                                                                                                                                                                        qobject.cpp                       2047 0x7ffff51c533e 
21 QWidget::~QWidget                                                                                                                                                                                     qwidget.cpp                       1517 0x7ffff63d9cb8 
22 QDesignerFormWindow::~QDesignerFormWindow                                                                                                                                                             qdesigner_formwindow.cpp          86   0x4a2a66       
23 QDesignerFormWindow::~QDesignerFormWindow                                                                                                                                                             qdesigner_formwindow.cpp          90   0x4a2a8a       
24 QObjectPrivate::deleteChildren                                                                                                                                                                        qobject.cpp                       2047 0x7ffff51c533e 
25 QWidget::~QWidget                                                                                                                                                                                     qwidget.cpp                       1517 0x7ffff63d9cb8 
26 QMdiSubWindow::~QMdiSubWindow                                                                                                                                                                         qmdisubwindow.cpp                 2256 0x7ffff65fadda 
27 QMdiSubWindow::~QMdiSubWindow                                                                                                                                                                         qmdisubwindow.cpp                 2263 0x7ffff65fadfc 
28 qDeleteInEventHandler                                                                                                                                                                                 qobject.cpp                       4691 0x7ffff51cc6c9 
29 QObject::event                                                                                                                                                                                        qobject.cpp                       1293 0x7ffff51c3ba5 
30 QWidget::event                                                                                                                                                                                        qwidget.cpp                       9061 0x7ffff63f0225 
31 QMdiSubWindow::event                                                                                                                                                                                  qmdisubwindow.cpp                 2891 0x7ffff65fd2ee 
32 QApplicationPrivate::notify_helper                                                                                                                                                                    qapplication.cpp                  3396 0x7ffff63747ba 
33 QApplication::notify                                                                                                                                                                                  qapplication.cpp                  3346 0x7ffff63745c2 
34 QCoreApplication::notifyInternal2                                                                                                                                                                     qcoreapplication.cpp              1040 0x7ffff4e7c4c4 
35 QCoreApplication::sendEvent                                                                                                                                                                           qcoreapplication.cpp              1442 0x7ffff4e7ce70 
36 QCoreApplicationPrivate::sendPostedEvents                                                                                                                                                             qcoreapplication.cpp              1801 0x7ffff4e7db92 
37 QCoreApplication::sendPostedEvents                                                                                                                                                                    qcoreapplication.cpp              1660 0x7ffff4e7d4ea 
38 postEventSourceDispatch                                                                                                                                                                               qeventdispatcher_glib.cpp         277  0x7ffff5191aa0 
39 g_main_context_dispatch                                                                                                                                                                                                                      0x7ffff43db96f 
40 g_main_context_iterate.constprop                                                                                                                                                                                                             0x7ffff442d758 
41 g_main_context_iteration                                                                                                                                                                                                                     0x7ffff43d8d43 
42 QEventDispatcherGlib::processEvents                                                                                                                                                                   qeventdispatcher_glib.cpp         427  0x7ffff51922f8 
43 QXcbGlibEventDispatcher::processEvents                                                                                                                                                                qxcbeventdispatcher.cpp           132  0x7fffe3fd0f06 
44 QEventLoop::processEvents                                                                                                                                                                             qeventloop.cpp                    139  0x7ffff4e8d623 
45 QEventLoop::exec                                                                                                                                                                                      qeventloop.cpp                    232  0x7ffff4e8d954 
46 QCoreApplication::exec                                                                                                                                                                                qcoreapplication.cpp              1355 0x7ffff4e7ccb2 
47 QGuiApplication::exec                                                                                                                                                                                 qguiapplication.cpp               1874 0x7ffff575eab2 
48 QApplication::exec                                                                                                                                                                                    qapplication.cpp                  2610 0x7ffff637183f 
49 main                                                                                                                                                                                                  main.cpp                          55   0x4800c9