Fix crash when manipulating disappearing elements

To reproduce:

• Start mock storage with two cubes, one is flickering
• Select the flickering cube
• Wait until it disappears
• Select the other cube -> crash

To fix: Interaction Dispatch must be notified when the selected element is deleted. Maybe add a method for this on the element base class, allowing Interaction Dispatch to watch a specific element. Should work both when single element is gone or entire layer is removed. If using the Godot notify system, add virtual override to allow child classes like Robot Element Node to still receive notifications.