Thanks a lot. Now I can create buttons with my own actions.
But I still have the problem that the SPLIT_WALL button disappears. Is it because "HomeView.ActionType.SPLIT_WALL" has no action? How do I combine this enum entry with an action?

I have only partially figured out how I add a button to the toolbar in the applet. After a lot of work, I managed to add a button without functionality. This button was really supposed to split walls. It disappears when I press another button. How do I add a button permanently to the toolbar, and ...