### 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)

int index = _sub_window_find(p_window);
	ERR_FAIL_COND(index == -1);
	const SubWindow &sw = gui.sub_windows[index];
	Transform2D pos;
	pos.set_origin(p_window->get_position());
	RS::get_singleton()->canvas_item_clear(sw.canvas_item);
	Rect2i r = Rect2i(p_window->get_position(), sw.window->get_size());
	if (!p_window->get_flag(Window::FLAG_BORDERLESS)) {
		Ref<StyleBox> panel = p_window->get_theme_stylebox(gui.subwindow_focused == p_window ? SNAME("embedded_border") : SNAME("embedded_unfocused_border"));
		panel->draw(sw.canvas_item, r);
		
		Ref<Font> title_font = p_window->get_theme_font(SNAME("title_font"));
		int font_size = p_window->get_theme_font_size(SNAME("title_font_size"));
		Color title_color = p_window->get_theme_color(SNAME("title_color"));
		int title_height = p_window->get_theme_constant(SNAME("title_height"));
		int close_h_ofs = p_window->get_theme_constant(SNAME("close_h_offset"));
		int close_v_ofs = p_window->get_theme_constant(SNAME("close_v_offset"));
		TextLine title_text = TextLine(p_window->atr(p_window->get_title()), title_font, font_size);
		title_text.set_width(r.size.width - panel->get_minimum_size().x - close_h_ofs);