### 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(p_event.is_null());
	Ref<InputEventMouseButton> mb = p_event;
	if (mb.is_valid()) {
		if (!mb->is_pressed()) {
			gui.mouse_focus_mask.clear_flag(mouse_button_to_mask(mb->get_button_index())); 
		}
	}
}
List<Control *>::Element *Viewport::_gui_add_root_control(Control *p_control) {
	gui.roots_order_dirty = true;
	return gui.roots.push_back(p_control);
}
void Viewport::gui_set_root_order_dirty() {
	ERR_MAIN_THREAD_GUARD;
	gui.roots_order_dirty = true;
}
void Viewport::_gui_force_drag(Control *p_base, const Variant &p_data, Control *p_control) {
	ERR_FAIL_COND_MSG(p_data.get_type() == Variant::NIL, "Drag data must be a value.");
	gui.dragging = true;
	gui.drag_data = p_data;
	gui.mouse_focus = nullptr;
	if (p_control) {
		_gui_set_drag_preview(p_base, p_control);
	}
	_propagate_viewport_notification(this, NOTIFICATION_DRAG_BEGIN);
}
void Viewport::_gui_set_drag_preview(Control *p_base, Control *p_control) {