Check if any custom notifications cause issues if target view is closed

There are quite a few custom notifications in use to communicate between views that need to be checked now that views are detached when they are not visible. One example of a problem is doubleclicking material in material browser when material editor is closed, it will open material editor empty.

Also check any showDockWidget() calls which open closed views, make sure they are done before whatever is then triggered in the opened view.