### Godot version

v4.0.beta.custom_build [0a9e6e478]

### System information

Ubuntu, Nvidia, Vulkan

### Issue description

When a Camera2D is freed - either explicitly or by being a child of a scene that is being switched away from via a method like `get_tree().change_scene_to_file()` - then this camera somehow stays active instead of Godot properly switching to no-camera-mode.

`get_viewport().get_camera_2d()` then returns all sorts of funny things from `<Freed Object>`  (it's supposed to return `null`) to arbitrary nodes of the current scene (not cameras).

This can lead to crashes.

### Steps to reproduce

checkout the MRP.

note that the "Go to scene 2" button switches places after pressing it.
that is because a camera is still active that does not exist in the current tree.

restart it a couple of times and spam the "Go to scene 2" button.
notice the garbage output.

### Minimal reproduction project

[godot_4_camera_bug.zip](https://github.com/godotengine/godot/files/10560455/godot_4_camera_bug.zip)
if (!click_on_window && r.has_point(mb->get_position())) {
				
				if (gui.subwindow_focused != sw.window) {
					
					_sub_window_grab_focus(sw.window);
				}
				click_on_window = true;
			}
			if (click_on_window) {
				break;
			}
		}
		if (!click_on_window && gui.subwindow_focused) {
			
			_sub_window_grab_focus(nullptr);
		}
	}
	if (gui.subwindow_focused) {
		Ref<InputEventMouseMotion> mm = p_event;
		if (mm.is_valid()) {
			SubWindowResize resize = _sub_window_get_resize_margin(gui.subwindow_focused, mm->get_position());
			if (resize != SUB_WINDOW_RESIZE_DISABLED) {
				DisplayServer::CursorShape shapes[SUB_WINDOW_RESIZE_MAX] = {
					DisplayServer::CURSOR_ARROW,
					DisplayServer::CURSOR_FDIAGSIZE,
					DisplayServer::CURSOR_VSIZE,
					DisplayServer::CURSOR_BDIAGSIZE,
					DisplayServer::CURSOR_HSIZE,
					DisplayServer::CURSOR_HSIZE,
					DisplayServer::CURSOR_BDIAGSIZE,