fix some interface assertion and nil bugs, and album page not showing now playing

ui/browsing/albumpage.go

@@ -130,7 +130,9 @@ func (a *AlbumPage) Route() controller.Route {
 	return controller.AlbumRoute(a.albumID)
 }
 
-func (a *AlbumPage) OnSongChange(track, lastScrobbledIfAny *mediaprovider.Track) {
+var _ CanShowNowPlaying = (*AlbumPage)(nil)
+
+func (a *AlbumPage) OnSongChange(track mediaprovider.MediaItem, lastScrobbledIfAny *mediaprovider.Track) {
 	a.nowPlayingID = sharedutil.MediaItemIDOrEmptyStr(track)
 	a.tracklist.SetNowPlaying(a.nowPlayingID)
 	a.tracklist.IncrementPlayCount(sharedutil.MediaItemIDOrEmptyStr(lastScrobbledIfAny))

ui/browsing/nowplayingpage.go

@@ -273,6 +273,9 @@ func (a *NowPlayingPage) onImageLoaded(img image.Image, err error) {
 		log.Printf("error loading cover art: %v\n", err)
 		return
 	}
+	if img == nil {
+		return
+	}
 	a.card.SetCoverImage(img)
 	c := dominantcolor.Find(img)
 	if c == a.background.StartColor {