### 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)

if (is_embedding_subwindows()) {
		for (int i = gui.sub_windows.size() - 1; i >= 0; i--) {
			Window *sw = gui.sub_windows[i].window;
			Rect2 swrect = Rect2(sw->get_position(), sw->get_size());
			Rect2 swrect_border = swrect;
			if (!sw->get_flag(Window::FLAG_BORDERLESS)) {
				int title_height = sw->get_theme_constant(SNAME("title_height"));
				int margin = sw->get_theme_constant(SNAME("resize_margin"));
				swrect_border.position.y -= title_height + margin;
				swrect_border.size.y += title_height + margin * 2;
				swrect_border.position.x -= margin;
				swrect_border.size.x += margin * 2;
			}
			if (swrect_border.has_point(p_pos)) {
				if (gui.mouse_over) {
					_drop_mouse_over();
				} else if (!gui.subwindow_over) {
					_drop_physics_mouseover();
				}
				if (swrect.has_point(p_pos)) {
					if (sw != gui.subwindow_over) {
						if (gui.subwindow_over) {
							gui.subwindow_over->_mouse_leave_viewport();
						}