Dialog has this clever way of re-sending a mouse event inside the bounardies of the dialog, so despite visual padding, the items can be clicked on any corner following Fitt's law.
However, when the mainItem has no size, adjusting the position will adjust it inside the border since there is no pixel inside the main item, leading to infinite recursion as the event is processed again and again and again leading to a crash.

Opened a Dialog with a mainItem without a width, dialog showed up tiny, hovered it, no longer crashed
Verified that event is still forwarded in the normal case.

I bet there's a tonne of crash reports like this (crash in Dialog::event in recursion) on bugzilla since an empty dialog could happen in case of a failure to load some QML bits.