Blender Git Loki

Fix T73191: Buttons in lower left of Preferences broken

I'm still not entirely sure what was going on - I know that the execute
region didn't get initialized correctly, but doing that at a later point
didn't fix the issue. Apparently forcing the header region to
re-initialize does fix it, even though I was sure this was redundant.

Also fixes a memory leak in UI code after preferences were opened.