Fix a number of crashes that affect Gimp 2.99
Continuing set of fixes related to problems with the font decorator in Gimp 2.99 when adding a text box into the canvas and selecting fonts on MacOS (Quartz).
(GTK versions of) NSWindows and NSViews appear to be getting lost track of.
Continuing fixes to: gimp#7608 (closed)
- The attempt here is to issue a few warnings, due to the fact that there still issues going on here.
- Try to add a little more deliberation to clearing out variables that end up otherwise holding bad data
- Fix a few occasions where an object of incorrect type is being cast incorrectly