### 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_THREAD_GUARD;
	gui.embed_subwindows_hint = p_embed;
}
bool Viewport::is_embedding_subwindows() const {
	ERR_READ_THREAD_GUARD_V(false);
	return gui.embed_subwindows_hint;
}
TypedArray<Window> Viewport::get_embedded_subwindows() const {
	TypedArray<Window> windows;
	for (int i = 0; i < gui.sub_windows.size(); i++) {
		windows.append(gui.sub_windows[i].window);
	}
	return windows;
}
void Viewport::subwindow_set_popup_safe_rect(Window *p_window, const Rect2i &p_rect) {
	int index = _sub_window_find(p_window);
	ERR_FAIL_COND(index == -1);
	gui.sub_windows.write[index].parent_safe_rect = p_rect;
}
Rect2i Viewport::subwindow_get_popup_safe_rect(Window *p_window) const {
	int index = _sub_window_find(p_window);
	
	
	if (index == -1) {
		return Rect2i();
	}
	
	return gui.sub_windows[index].parent_safe_rect;
}
void Viewport::pass_mouse_focus_to(Viewport *p_viewport, Control *p_control) {