### 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)
if (viewport_under) {
				if (viewport_under != this) {
					Transform2D ai = viewport_under->get_final_transform().affine_inverse();
					viewport_pos = ai.xform(viewport_pos);
				}
				
				gui.drag_mouse_over = viewport_under->gui_find_control(viewport_pos);
				if (gui.drag_mouse_over) {
					Transform2D localizer = gui.drag_mouse_over->get_global_transform_with_canvas().affine_inverse();
					gui.drag_mouse_over_pos = localizer.xform(viewport_pos);
					bool can_drop = _gui_drop(gui.drag_mouse_over, gui.drag_mouse_over_pos, true);
					if (!can_drop) {
						ds_cursor_shape = DisplayServer::CURSOR_FORBIDDEN;
					} else {
						ds_cursor_shape = DisplayServer::CURSOR_CAN_DROP;
					}
				}
			} else {
				gui.drag_mouse_over = nullptr;
			}
		}
		if (DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_CURSOR_SHAPE)) {
			DisplayServer::get_singleton()->cursor_set_shape(ds_cursor_shape);
		}
	}
	Ref<InputEventScreenTouch> touch_event = p_event;
	if (touch_event.is_valid()) {
		Size2 pos = touch_event->get_position();
		const int touch_index = touch_event->get_index();
		if (touch_event->is_pressed()) {