Merge pull request TeamNewPipe#11651 from u7656655/fix-addtoplaylist-crash

Fix crash after adding item to a playlist caused by null thumbnail URL

Author: TobiGr

app/src/main/java/org/schabi/newpipe/local/dialog/PlaylistAppendDialog.java

@@ -160,7 +160,8 @@ private void onPlaylistSelected(@NonNull final LocalPlaylistManager manager,
                 .subscribe(ignored -> {
                     successToast.show();
 
-                    if (playlist.thumbnailUrl.equals(PlaylistEntity.DEFAULT_THUMBNAIL)) {
+                    if (playlist.thumbnailUrl != null
+                            && playlist.thumbnailUrl.equals(PlaylistEntity.DEFAULT_THUMBNAIL)) {
                         playlistDisposables.add(manager
                                 .changePlaylistThumbnail(playlist.getUid(), streams.get(0).getUid(),
                                         false)