### 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
if (p_tree && !p_tree->is_connected("animation_player_changed", callable_mp(this, &AnimationTreeEditor::_animation_list_changed))) {
		p_tree->connect("animation_player_changed", callable_mp(this, &AnimationTreeEditor::_animation_list_changed), CONNECT_DEFERRED);
	}
	if (tree == p_tree) {
		return;
	}
	if (tree && tree->is_connected("animation_player_changed", callable_mp(this, &AnimationTreeEditor::_animation_list_changed))) {
		tree->disconnect("animation_player_changed", callable_mp(this, &AnimationTreeEditor::_animation_list_changed));
	}
	tree = p_tree;
	Vector<String> path;
	if (tree && tree->has_meta("_tree_edit_path")) {
		path = tree->get_meta("_tree_edit_path");
	} else {
		current_root = ObjectID();
	}
	edit_path(path);
}
void AnimationTreeEditor::_node_removed(Node *p_node) {