[5.8.0 RC] QtDesigner crashes at close up

Thread 1 "designer" received signal SIGSEGV, Segmentation fault.
0x0000000000433a00 in QDesignerWorkbench::formWindowCount() const ()
(gdb) bt
#0  0x0000000000433a00 in QDesignerWorkbench::formWindowCount() const ()
#1  0x0000000000432618 in QDesignerFormWindow::getNumberOfUntitledWindows() const ()
#2  0x0000000000432998 in QDesignerFormWindow::updateWindowTitle(QString const&) ()
#3  0x0000000000432e0a in QDesignerFormWindow::updateChanged() ()
#4  0x00007ffff5536a86 in QMetaObject::activate(QObject*, int, int, void**) ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Core.so.5
#5  0x00007ffff6c1341e in QUndoStack::indexChanged(int) ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Widgets.so.5
#6  0x00007ffff6c148e2 in QUndoStack::clear() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Widgets.so.5
#7  0x00007ffff6c149e2 in QUndoStack::~QUndoStack() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Widgets.so.5
#8  0x00007ffff7ab24d7 in qdesigner_internal::FormWindow::~FormWindow() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5DesignerComponents.so.5
#9  0x00007ffff7ab2559 in qdesigner_internal::FormWindow::~FormWindow() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5DesignerComponents.so.5
#10 0x00007ffff7abb536 in qdesigner_internal::FormWindowManager::~FormWindowManager() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5DesignerComponents.so.5
#11 0x00007ffff7abb589 in qdesigner_internal::FormWindowManager::~FormWindowManager() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5DesignerComponents.so.5
#12 0x00007ffff72e7bb3 in QDesignerFormEditorInterfacePrivate::~QDesignerFormEditorInterfacePrivate() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Designer.so.5
#13 0x00007ffff72e7be8 in QDesignerFormEditorInterface::~QDesignerFormEditorInterface() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Designer.so.5
#14 0x00007ffff7ac1897 in qdesigner_internal::FormEditor::~FormEditor() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5DesignerComponents.so.5
#15 0x00007ffff5534e53 in QObjectPrivate::deleteChildren() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Core.so.5
#16 0x00007ffff553e0d4 in QObject::~QObject() ()
   from /home/milla2/tmp/qt-everywhere-enterprise-src-5.8.0-rc/qtbase/lib/libQt5Core.so.5
#17 0x0000000000434639 in QDesignerWorkbench::~QDesignerWorkbench() ()
#18 0x000000000042e8c1 in QDesigner::~QDesigner() ()
#19 0x000000000041deab in main ()