@KH-219Design
thank you， I‘ll try some tests.

Update: I downloaded the most recent version of Qt, 5.13.0, and I could not get the issue to repeat. So I think the problem is related to specific versions of Qt.

@kshegunov @mrjj

Thanks for your explanation. Finally I figured out that the problem lay in the signal QGraphicsScene::changed I used. I learned about this signal in an OpenGL example in Qt. By using it I was trying to update my wave soon after its last update, however this signal could also be triggered by changing axis range or window resize, which would probably cause the crash for I didn't consider thread safety and synchronization under this condition.

So I defined a custom signal function and now the problem is solved.

@kshegunov Thank you for your suggestion I will try to follow it.

main.qml - What is the top level item ? Is it window or Item ?
loginform.qml - What is top level item ? Is it window or Item ?

This is typical issue of object deleted already is getting delete due to parent child relation ship ?