Mac OS (tested on Catalina and Mojave) allows to raise child dialog over parent dialog also it allows closing parent dialog even child dialog is still executed (looks like in Qt 5.12.x there is no this issue). It freezes application.

For ex. steps how to reproduce it in Qt Creator 4.12.2:

run Qt Creator

go to preferences

in Debugger page click "Add Qt sources" (folder selection dialog is raised)

Click on Preferences page (the page raised)

Close Preferences page (folder selection dialog is still executed)

Close folder selection dialog

Qt Creator Frozen

As workaround ** all child dialogues must be WindowModal as default

Properly fix: find a way how to prevent closing parent dialog (there is no hide or close event on click close button, so using event filter is not helpful)