Fix before exit crash in ProjectTree

When there are 2 top level projects in session, crash happens in
foldersAboutToBeRemoved as currentNode() not properly updated and it
points to previously unloaded dangling node.

Change-Id: Ie3f88bdd03ec042ff1cce6712ed923465c902701
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>