### 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
state_machine_draw->queue_redraw();
					break;
				}
				bool acstate = transition_lines[i].advance_condition_name != StringName() && bool(tree->get(AnimationTreeEditor::get_singleton()->get_base_path() + String(transition_lines[i].advance_condition_name)));
				if (transition_lines[i].advance_condition_state != acstate) {
					state_machine_draw->queue_redraw();
					break;
				}
			}
			bool same_travel_path = true;
			Vector<StringName> tp;
			bool is_playing = false;
			StringName current_node;
			StringName fading_from_node;
			current_play_pos = 0;
			current_length = 0;
			fade_from_current_play_pos = 0;
			fade_from_length = 0;
			fading_time = 0;
			fading_pos = 0;
			if (playback.is_valid()) {
				tp = playback->get_travel_path();
				is_playing = playback->is_playing();
				current_node = playback->get_current_node();
				fading_from_node = playback->get_fading_from_node();
				current_play_pos = playback->get_current_play_pos();
				current_length = playback->get_current_length();
				fade_from_current_play_pos = playback->get_fade_from_play_pos();
				fade_from_length = playback->get_fade_from_length();
				fading_time = playback->get_fading_time();