DiapDealer,
From searching, I do not think it is anything Sigil is doing per-se. Although please remove that m_pluginMapper from MainWindow.h and from the macOS specific delete in the destructor, just to be safe.
Based on a long search of bug reports, I think the bug that shows it happening is here:
https://bugreports.qt.io/browse/QTBUG-75326
I have tested a number of Qt patches and only one seems to help but it was abandoned by Qt due to potential other difficulties it may cause.
That said the clear platfromWindow pointer early in destroy() patch seems to prevent the problem with that issue's test case that the earlier workaround did not really fix, it just prevented a a specific type of crash. We are seeing other crashes in different areas related to modal dialogs and mainwindows being deleted when they had focus causing focus to get "lost" and things start to mess up.
This only impacts macOS and it is hard to hit as well and can be recreated even in Qt 5.12.3 and earlier Sigil builds.
KevinH