### 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)
}
		BitField<MouseButtonMask> mask = gui.mouse_focus_mask;
		Point2 click = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(gui.last_mouse_pos);
		for (int i = 0; i < 3; i++) {
			if ((int)mask & (1 << i)) {
				Ref<InputEventMouseButton> mb;
				mb.instantiate();
				
				mb->set_position(click);
				mb->set_button_index(MouseButton(i + 1));
				mb->set_pressed(false);
				gui.mouse_focus->_call_gui_input(mb);
			}
		}
		gui.mouse_focus = focus_grabber;
		click = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(gui.last_mouse_pos);
		for (int i = 0; i < 3; i++) {
			if ((int)mask & (1 << i)) {
				Ref<InputEventMouseButton> mb;
				mb.instantiate();
				
				mb->set_position(click);
				mb->set_button_index(MouseButton(i + 1));
				mb->set_pressed(true);
				MessageQueue::get_singleton()->push_callable(callable_mp(gui.mouse_focus, &Control::_call_gui_input), mb);
			}
		}
	}
}
void Viewport::push_text_input(const String &p_text) {