Segmentation fault from null pointer dereference in mapToGlobalTransform

This was originally reported downstream at KDE, but they agree that this is likely a bug in Qt, not KDE software.

Steps to reproduce:

1. Use Kate (KDE text editor) to create a reStructuredText document with some headings.
2. Collapse the headings.
3. Continue to move mouse over the > in the left side bar where the line numbers are, showing the folded content in a tooltip.

Backtrace:

Thread 1 "kate" received signal SIGSEGV, Segmentation fault.
QGraphicsItem::scene (this=0x28) at graphicsview/qgraphicsitem.cpp:1585
1585 graphicsview/qgraphicsitem.cpp: No such file or directory.
(gdb) bt
#0 QGraphicsItem::scene (this=0x28) at graphicsview/qgraphicsitem.cpp:1585
#1 0x00003ffff6902520 in mapToGlobalTransform (w=0x100cde180) at kernel/qwidget.cpp:12532
#2 0x00003ffff690275c in QWidget::mapFromGlobal (this=<optimized out>, pos=...) at kernel/qwidget.cpp:12582
#3 0x00003ffff68bac8c in QApplicationPrivate::dispatchEnterLeave (enter=0x100cde180, leave=<optimized out>, globalPosF=...) at kernel/qapplication.cpp:2351
#4 0x00003ffff68bb880 in QApplicationPrivate::sendMouseEvent (receiver=0x100cde180, event=0x3fffffffe200, alienWidget=0x0, nativeWidget=0x100cde180, buttonDown=0x3ffff6f0c460 <qt_button_down>, lastMouseReceiver=..., spontaneous=<optimized out>)
at kernel/qapplication.cpp:2684
#5 0x00003ffff6943cb8 in QWidgetWindow::handleMouseEvent (this=0x100d036a0, event=0x3fffffffe738) at kernel/qwidgetwindow.cpp:629
#6 0x00003ffff6946f68 in QWidgetWindow::event (this=0x100d036a0, event=0x3fffffffe738) at kernel/qwidgetwindow.cpp:250
#7 0x00003ffff68b12f4 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x100d036a0, e=0x3fffffffe738) at kernel/qapplication.cpp:3722
#8 0x00003ffff68bcba0 in QApplication::notify (this=0x3fffffffef20, receiver=0x100d036a0, e=0x3fffffffe738) at kernel/qapplication.cpp:3094
#9 0x00003ffff5c07010 in QCoreApplication::notifyInternal2 (receiver=0x100d036a0, event=0x3fffffffe738) at kernel/qcoreapplication.cpp:1024
#10 0x00003ffff6233b58 in QCoreApplication::sendSpontaneousEvent (event=0x3fffffffe728, receiver=0x100d036a0) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:236
#11 QGuiApplicationPrivate::processMouseEvent (e=0x100abcc80) at kernel/qguiapplication.cpp:1952
#12 0x00003ffff6235f60 in QGuiApplicationPrivate::processWindowSystemEvent (e=<optimized out>) at kernel/qguiapplication.cpp:1733
#13 0x00003ffff61f6d64 in QWindowSystemInterface::sendWindowSystemEvents (flags=...) at kernel/qwindowsysteminterface.cpp:946
#14 0x00003ffff1924314 in userEventSourceDispatch (source=<optimized out>) at qeventdispatcher_glib.cpp:77
#15 0x00003ffff3c5c2a0 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#16 0x00003ffff3c5c5e8 in ?? () from /usr/lib/libglib-2.0.so.0
#17 0x00003ffff3c5c71c in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#18 0x00003ffff5c8a5b4 in QEventDispatcherGlib::processEvents (this=0x3ffff3c72260 <g_poll>, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#19 0x00003ffff19245f0 in QPAEventDispatcherGlib::processEvents (this=<optimized out>, flags=...) at qeventdispatcher_glib.cpp:122
#20 0x00003ffff5c03a14 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:134
#21 0x00003ffff5c041f0 in QEventLoop::exec (this=0x3fffffffeca8, flags=...) at kernel/qeventloop.cpp:212
#22 0x00003ffff5c11fa8 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1297
#23 0x00003ffff6228118 in QGuiApplication::exec () at kernel/qguiapplication.cpp:1679
#24 0x00003ffff68b1100 in QApplication::exec () at kernel/qapplication.cpp:2910
#25 0x00000001000314e8 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/packages/user/kate/src/kate-18.04.3/kate/main.cpp:595

Further display information:

Version
=======
KWin version: 5.12.6
Qt Version: 5.9.6
Qt compile version: 5.9.6
XCB compile version: 1.13

Operation Mode: X11 only

X11
===
Vendor: The X.Org Foundation
Vendor Release: 12000000
Protocol Version/Revision: 11/0
SHAPE: yes; Version: 0x11
RANDR: yes; Version: 0x14
DAMAGE: yes; Version: 0x11
Composite: yes; Version: 0x4
RENDER: yes; Version: 0xb
XFIXES: yes; Version: 0x50
SYNC: yes; Version: 0x31
GLX: yes; Version: 0x0

Screens
=======
Multi-Head: no
Active screen follows mouse: yes
Number of Screens: 1

Screen 0:
---------
Name: DVI-0
Geometry: 0,0,1920x1080
Scale: 1
Refresh Rate: 60

Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: X.Org
OpenGL renderer string: AMD CAICOS (DRM 2.50.0 / 4.14.56-mc9-easy, LLVM 6.0.1)
OpenGL version string: 3.1 Mesa 18.1.3
OpenGL platform interface: GLX
OpenGL shading language version string: 1.40
Driver: R600G
GPU class: Unknown
OpenGL version: 3.1
GLSL version: 1.40
Mesa version: 18.1.3
X server version: 1.20
Linux kernel version: 4.14.56
Direct rendering: Requires strict binding: no
GLSL shaders: yes
Texture NPOT support: yes
Virtual Machine: no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace: no