Fix terrain selection crash
See merge request OpenMW/openmw!1165
(cherry picked from commit ec4e3b04a7)
b84e41bd Avoid storing ref, dynamic cast worldspacewidget for safety
d62ddc00 Use QPointer to detect object existence, less verbose debug messages
cb42b528 Remove friend, make getEditMode public to allow editmode testing.
4b148180 Restucture code
08f7c73e Fix text