### 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)
gui.forced_mouse_focus = false;
			}
			bool stopped = false;
			if (mouse_focus && mouse_focus->can_process()) {
				stopped = _gui_call_input(mouse_focus, mb);
			}
			if (stopped) {
				set_input_as_handled();
			}
		}
	}
	Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid()) {
		gui.key_event_accepted = false;
		Point2 mpos = mm->get_position();
		
		if (!gui.drag_attempted && gui.mouse_focus && (mm->get_button_mask().has_flag(MouseButtonMask::LEFT))) {
			gui.drag_accum += mm->get_relative();
			float len = gui.drag_accum.length();
			if (len > 10) {
				{ 
					CanvasItem *ci = gui.mouse_focus;
					while (ci) {
						Control *control = Object::cast_to<Control>(ci);
						if (control) {
							gui.dragging = true;
							gui.drag_data = control->get_drag_data(control->get_global_transform_with_canvas().affine_inverse().xform(mpos - gui.drag_accum));
							if (gui.drag_data.get_type() != Variant::NIL) {
								gui.mouse_focus = nullptr;
								gui.forced_mouse_focus = false;