### 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
AnimationTree *tree = AnimationTreeEditor::get_singleton()->get_animation_tree();
	if (!tree) {
		return;
	}
	Ref<AnimationNodeStateMachinePlayback> playback = tree->get(AnimationTreeEditor::get_singleton()->get_base_path() + "playback");
	if (!playback.is_valid() || !playback->is_playing()) {
		return;
	}
	{
		float len = MAX(0.0001, current_length);
		float pos = CLAMP(current_play_pos, 0, len);
		float c = pos / len;
		_state_machine_pos_draw_individual(playback->get_current_node(), c);
	}
	{
		float len = MAX(0.0001, fade_from_length);
		float pos = CLAMP(fade_from_current_play_pos, 0, len);
		float c = pos / len;
		_state_machine_pos_draw_individual(playback->get_fading_from_node(), c);
	}
}
void AnimationNodeStateMachineEditor::_update_graph() {