Application: okular (0.26.1)
KDE Platform Version: 4.14.26
Qt Version: 4.8.7
Operating System: Linux 4.14.0-0.bpo.3-amd64 x86_64
Distribution: Debian GNU/Linux 9.4 (stretch)

-- Information about the crash:
- What I was doing when the application crashed:

Okular was open with a dokument over night. This time several nights last time just one night. 

Possibly "helping" condition was that I had a lot in memory  and parts of okular where possibly swapped out.

The crash can be reproduced sometimes.

-- Backtrace:
Application: Okular (okular), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[KCrash Handler]
#6  QWidgetPrivate::childAtRecursiveHelper (this=0x55795d26d390, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10012
#7  0x00007f82a8c23280 in QWidgetPrivate::childAtRecursiveHelper (this=0x55795d21ef60, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10033
#8  0x00007f82a8c23280 in QWidgetPrivate::childAtRecursiveHelper (this=0x55795d1a8da0, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10033
#9  0x00007f82a8c23280 in QWidgetPrivate::childAtRecursiveHelper (this=0x55795d1971a0, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10033
#10 0x00007f82a8c23280 in QWidgetPrivate::childAtRecursiveHelper (this=0x55795d3c6b10, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10033
#11 0x00007f82a8c23280 in QWidgetPrivate::childAtRecursiveHelper (this=this@entry=0x55795d258ff0, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false, includeFrame=includeFrame@entry=false) at kernel/qwidget.cpp:10033
#12 0x00007f82a8c23381 in QWidgetPrivate::childAt_helper (this=0x55795d258ff0, p=..., ignoreChildrenInDestructor=ignoreChildrenInDestructor@entry=false) at kernel/qwidget.cpp:10001
#13 0x00007f82a8c233eb in QWidget::childAt (this=this@entry=0x55795d29f910, p=...) at kernel/qwidget.cpp:9981
#14 0x00007f82a8c5cadf in QApplication::x11ProcessEvent (this=0x7ffc0b6f5400, event=event@entry=0x7ffc0b6f4f80) at kernel/qapplication_x11.cpp:3660
#15 0x00007f82a8c86502 in x11EventSourceDispatch (s=0x55795ceb9c00, callback=0x0, user_data=0x0) at kernel/qguieventdispatcher_glib.cpp:146
#16 0x00007f82a45bd7f7 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#17 0x00007f82a45bda60 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#18 0x00007f82a45bdb0c in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#19 0x00007f82a81ec854 in QEventDispatcherGlib::processEvents (this=0x55795ce8ccf0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#20 0x00007f82a8c865d6 in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=...) at kernel/qguieventdispatcher_glib.cpp:204
#21 0x00007f82a81ba7ef in QEventLoop::processEvents (this=this@entry=0x7ffc0b6f5360, flags=...) at kernel/qeventloop.cpp:149
#22 0x00007f82a81bab55 in QEventLoop::exec (this=this@entry=0x7ffc0b6f5360, flags=...) at kernel/qeventloop.cpp:204
#23 0x00007f82a81c0bd9 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1227
#24 0x000055795b6722a3 in main (argc=<optimized out>, argv=<optimized out>) at ./shell/main.cpp:64

Reported using DrKonqi