### 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)
bool stopped = false;
			if (gui.mouse_focus && gui.mouse_focus->can_process()) {
				stopped = _gui_call_input(gui.mouse_focus, mb);
			}
			if (stopped) {
				set_input_as_handled();
			}
			if (gui.dragging && mb->get_button_index() == MouseButton::LEFT) {
				
				_perform_drop(gui.mouse_focus, pos);
			}
			_gui_cancel_tooltip();
		} else {
			if (gui.dragging && mb->get_button_index() == MouseButton::LEFT) {
				_perform_drop(gui.drag_mouse_over, gui.drag_mouse_over_pos);
			}
			gui.mouse_focus_mask.clear_flag(mouse_button_to_mask(mb->get_button_index())); 
			if (!gui.mouse_focus) {
				
				return;
			}
			mb = mb->xformed_by(Transform2D()); 
			Point2 pos = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(mpos);
			mb->set_position(pos);
			Control *mouse_focus = gui.mouse_focus;
			
			
			
			if (gui.mouse_focus_mask.is_empty()) {
				gui.mouse_focus = nullptr;