This was caused by accepting the call, which caused the notification to
be removed. When `active_call.accept_incoming()` finally completed, we
would try to get the app state from it in order to join the project, but
couldn't becuase the view would have already been dropped.

This commit fixes the bug by capturing a weak handle to the app state
when accepting the call as opposed to trying to read it from the view
when the accept completes.