### 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)
ERR_FAIL_COND_V(!is_inside_tree(), nullptr);
	Viewport *v = const_cast<Viewport *>(this);
	Window *w = Object::cast_to<Window>(v);
	while (!w) {
		v = v->get_parent_viewport();
		w = Object::cast_to<Window>(v);
	}
	return w;
}
void Viewport::_gui_remove_focus_for_window(Node *p_window) {
	if (get_base_window() == p_window) {
		gui_release_focus();
	}
}
bool Viewport::_gui_control_has_focus(const Control *p_control) {
	return gui.key_focus == p_control;
}
void Viewport::_gui_control_grab_focus(Control *p_control) {
	if (gui.key_focus && gui.key_focus == p_control) {
		
		return;
	}
	get_tree()->call_group("_viewports", "_gui_remove_focus_for_window", (Node *)get_base_window());
	gui.key_focus = p_control;
	emit_signal(SNAME("gui_focus_changed"), p_control);
	p_control->notification(Control::NOTIFICATION_FOCUS_ENTER);
	p_control->queue_redraw();
}
void Viewport::_gui_accept_event() {