### Godot version

41e5f5b0930343ef548a56d553ba20fafd672147

### System information

Windows 10, Vulkan, Intel® Core™ i7-8700K, Nvidia GTX 1080 (471.11)

### Issue description

The editor will crash if you select an AnimationTree node then select another node. This seems to be caused by the edit function assigning a null object via the edit interface, then calling another function which does not appear to be null.

### Steps to reproduce

Create an AnimationTree node then select another node.

### Minimal reproduction project

N/A
Transition tr;
	tr.from = p_from;
	tr.to = p_to;
	tr.transition = p_transition;
	transitions.push_back(tr);
}
bool EditorAnimationMultiTransitionEdit::_set(const StringName &p_name, const Variant &p_property) {