When I try to run it,the System crashes.
Nothing will happen,because the UE4 has been forced out.
Destroy Actor ‘Target is System UI’
You are destroying your UI handling system, not the widget. That’s why it crashes.
There is no explicit DestroyWidget call for UMG. Assuming you actually want to destroy your widget, and not just turn it invisible, you should get the ‘parent widget’ and call ClearChildren on that.