Reproducing the bug:
1) Download Qt 5.15 

2) Compile the code from documentation https://doc.qt.io/qt-5/qml-qtquick-dialogs-colordialog.html 

3) Click on OK/Cancel buttons (or close the dialog in other ways)

4) ColorDialog cannot be closed and the whole app does not respond to any actions.

This is caused because a few files are missing in qt sources for android. Manually adding DefaultWindowDecoration.qml  (easily found on the Internet) to

~/ Qt/5.15.0/android/qml/QtQuick/Dialogs/

fixes the problem, but causes 3 another warning, anyway everything works fine.