### 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
int index = String(p_name).get_slicec('/', 0).to_int();
	StringName prop = String(p_name).get_slicec('/', 1);
	bool found;
	transitions.write[index].transition->set(prop, p_property, &found);
	if (found) {
		return true;
	}
	return false;
}
bool EditorAnimationMultiTransitionEdit::_get(const StringName &p_name, Variant &r_property) const {
	int index = String(p_name).get_slicec('/', 0).to_int();
	StringName prop = String(p_name).get_slicec('/', 1);
	if (prop == "transition_path") {
		r_property = String(transitions[index].from) + " -> " + transitions[index].to;
		return true;
	}
	bool found;
	r_property = transitions[index].transition->get(prop, &found);
	if (found) {
		return true;
	}
	return false;
}
void EditorAnimationMultiTransitionEdit::_get_property_list(List<PropertyInfo> *p_list) const {