Crash triggered by QQuickWindowPrivate::deliverPressOrReleaseEvent

We faced a problem that leads to application crash while performing some touch operations on touchscreen (e.g. sliding screen left to right).

The problem is caused by unsafe memory access in QQuickWindowPrivate while it is handling touchEvent.

I made some analysis according to stack trace and found following lines (https://github.com/qt/qtdeclarative/blob/5.12.1/src/quick/items/qquickwindow.cpp line 2530): 

if (isTouchEvent && event->isPressEvent() && isDeliveringTouchAsMouse() && pointerEventInstance(touchMouseDevice)->pointById(touchMouseId)->grabberPointerHandler()) {

 It seems like after calling pointById(touchMouseId) we are receiving nullptr and after that app crashed in call of grabberPointerHandler() with this=nullptr instance object.

It's hard for us to add simple example for it (seems like that nullptr access problem is very rare) but I pointed the place with unsafe memory access.

I also attached gdb stack trace output.