### 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.key_event_accepted = true;
	if (is_inside_tree()) {
		set_input_as_handled();
	}
}
void Viewport::_drop_mouse_over() {
	if (gui.mouse_over) {
		_gui_call_notification(gui.mouse_over, Control::NOTIFICATION_MOUSE_EXIT);
		gui.mouse_over = nullptr;
	}
}
void Viewport::_drop_mouse_focus() {
	Control *c = gui.mouse_focus;
	BitField<MouseButtonMask> mask = gui.mouse_focus_mask;
	gui.mouse_focus = nullptr;
	gui.forced_mouse_focus = false;
	gui.mouse_focus_mask.clear();
	for (int i = 0; i < 3; i++) {
		if ((int)mask & (1 << i)) {
			Ref<InputEventMouseButton> mb;
			mb.instantiate();
			mb->set_position(c->get_local_mouse_position());
			mb->set_global_position(c->get_local_mouse_position());
			mb->set_button_index(MouseButton(i + 1));
			mb->set_pressed(false);
			c->_call_gui_input(mb);
		}
	}
}
void Viewport::_drop_physics_mouseover(bool p_paused_only) {