this error occurs if you did not define an itemEditor and is a bug actually (as the LevelEditor should work also without an ItemEditor), so thanks a lot for reporting! It will be fixed with the next engine update.
To solve this bug in the meantime, please either
1.) add an ItemEditor to your game and set its id to itemEditor (you can leave the contents of the ItemEditor empty); or alternatively you can
2.) set the itemEditorItem property of LevelEditor to undefined.
Please let me know if that solved it for you,