We can open we main menu of systemsettings the first time but it doesn’t work afterwards until restart of the application.
STEPS TO REPRODUCE
1. Open the main menu of systemsettings
2. Close it
3. Try to reopen it
Doesn’t open a second time. That appears in the terminal output:
file:///usr/lib/qt/qml/QtQuick/Controls/Private/MenuContentItem.qml:239: TypeError: Cannot read property 'submenuPopupDelay' of null
KDE Plasma Version: 5.14.3
KDE Frameworks Version: 5.52.0
Qt Version: 5.12.0
The problem appears to be because of the update to Qt 5.12.
*** This bug has been marked as a duplicate of bug 399914 ***