Backport memory cleanup from 2.5 to 2.4

[vkbo]

The memory cleanup changes made in 2.5 probably should be backported to 2.4. The old implementation does cause crashes occasionally when closing the Manuscript Build tool too early (before the document was rendered) due to it being triggered by a timer.

This timer does not exist in 2.4, but there is a timer that triggers the update of the yellow banner that also can crash if the dialog is closed while it's being updated. This is a tiny target window to hit, since it is very quick, but I have managed to hit it.