Reincarnate the last layer even if there is a "fake" reference layer

Otherwise there is a GUI lock happens:

1. Add a reference image
2. Remove all the layers in the layers docker

(and now you cannot add any new layers :) )

BACKPORT:krita/4.2
(cherry picked from commit 555ad15f39cddf7b4342bacfd1ae447d5fa1f1f6)