### 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
#include "animation_state_machine_editor.h"
#include "core/config/project_settings.h"
#include "core/input/input.h"
#include "core/io/resource_loader.h"
#include "core/math/geometry_2d.h"
#include "core/os/keyboard.h"
#include "editor/editor_file_dialog.h"
#include "editor/editor_node.h"
#include "editor/editor_scale.h"
#include "editor/editor_settings.h"
#include "editor/editor_undo_redo_manager.h"
#include "scene/animation/animation_blend_tree.h"
#include "scene/animation/animation_player.h"
#include "scene/gui/menu_button.h"
#include "scene/gui/option_button.h"
#include "scene/gui/panel.h"
#include "scene/gui/panel_container.h"
#include "scene/gui/separator.h"
#include "scene/gui/tree.h"
#include "scene/main/viewport.h"
#include "scene/main/window.h"
bool AnimationNodeStateMachineEditor::can_edit(const Ref<AnimationNode> &p_node) {