### Godot version

4.0.3.stable

### System information

Windows 10/11

### Issue description

I want the player's cursor to change when highlighting pickable Area2Ds; to do this, I have the areas modify the `mouse_default_cursor_shape` property on an external Control when the mouse enters or exits. If another Control appears in front of the Area2D while the mouse is over it, there is a chance that the project will crash.

Nothing prints to the IDE's output on this crash. If running the editor with console, then
```
ERROR: Condition "slot >= slot_max" is true. Returning: nullptr
   at: get_instance (./core/object/object.h:972)
```
will sometimes appear alongside the crash. If it appears, it always appears twice.

When I run a debug build of the editor through WinDbg, WinDbg will catch the following exception:
```
(3b58.2aac): Access violation - code c0000005 (!!! second chance !!!)
godot_windows_editor_x86_64!ObjectDB::get_instance [inlined in godot_windows_editor_x86_64!Viewport::_cleanup_mouseover_colliders+0x6f]:
00007ff6`ac254a1f 488b5310        mov     rdx,qword ptr [rbx+10h] ds:cccccccc`c31c419b=????????????????
```
When using this method, the exception is not random; it always occurs on the first attempt.

### Steps to reproduce

- Open the minimal reproduction project in the editor.
- Run the project.
- Click on the Area2D (you'll want visible collision shapes on).
- A full-screen button will pop up. If the game does not crash at this point, clicking the button will hide it so you can try again. It should crash within a few seconds of mashing.

### Minimal reproduction project

[Crash Test.zip](https://github.com/godotengine/godot/files/12027772/Crash.Test.zip)

ERR_FAIL_COND(!is_inside_tree());
	for (int i = 0; i < gui.sub_windows.size(); i++) {
		ERR_FAIL_COND(gui.sub_windows[i].window == p_window);
	}
	if (gui.sub_windows.size() == 0) {
		subwindow_canvas = RS::get_singleton()->canvas_create();
		RS::get_singleton()->viewport_attach_canvas(viewport, subwindow_canvas);
		RS::get_singleton()->viewport_set_canvas_stacking(viewport, subwindow_canvas, SUBWINDOW_CANVAS_LAYER, 0);
	}
	SubWindow sw;
	sw.canvas_item = RS::get_singleton()->canvas_item_create();
	RS::get_singleton()->canvas_item_set_parent(sw.canvas_item, subwindow_canvas);
	sw.window = p_window;
	gui.sub_windows.push_back(sw);
	if (gui.subwindow_drag == SUB_WINDOW_DRAG_DISABLED) {
		if (p_window->get_flag(Window::FLAG_NO_FOCUS)) {
			_sub_window_update_order();
		} else {
			_sub_window_grab_focus(p_window);
		}
	} else {
		int index = _sub_window_find(gui.currently_dragged_subwindow);
		sw = gui.sub_windows[index];
		gui.sub_windows.remove_at(index);
		gui.sub_windows.push_back(sw);
		_sub_window_update_order();
	}
	RenderingServer::get_singleton()->viewport_set_parent_viewport(p_window->viewport, viewport);
}
void Viewport::_sub_window_update(Window *p_window) {