playlistview: Do not add a playlist if the view is not populated

Since commit 0faaeea7, the views are populated when opened. This means that a playlist can be created before the PlaylistView has been populated. In that case, the newly created playlist will be displayed two times: one because of the playlist creation logic and a second time because of the populate logic.

Fix the issue by doing nothing in PlaylistView on playlist creation if the sidebar is not populated. The playlist will be correctly displayed on populate.