### 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)
RenderingServer::get_singleton()->viewport_set_parent_viewport(viewport, RID());
		} break;
		case NOTIFICATION_INTERNAL_PHYSICS_PROCESS: {
			if (!get_tree()) {
				return;
			}
			if (get_tree()->is_debugging_collisions_hint() && contact_2d_debug.is_valid()) {
				RenderingServer::get_singleton()->canvas_item_clear(contact_2d_debug);
				RenderingServer::get_singleton()->canvas_item_set_draw_index(contact_2d_debug, 0xFFFFF); 
				Vector<Vector2> points = PhysicsServer2D::get_singleton()->space_get_contacts(find_world_2d()->get_space());
				int point_count = PhysicsServer2D::get_singleton()->space_get_contact_count(find_world_2d()->get_space());
				Color ccol = get_tree()->get_debug_collision_contact_color();
				for (int i = 0; i < point_count; i++) {
					RenderingServer::get_singleton()->canvas_item_add_rect(contact_2d_debug, Rect2(points[i] - Vector2(2, 2), Vector2(5, 5)), ccol);
				}
			}
#ifndef _3D_DISABLED
			if (get_tree()->is_debugging_collisions_hint() && contact_3d_debug_multimesh.is_valid()) {
				Vector<Vector3> points = PhysicsServer3D::get_singleton()->space_get_contacts(find_world_3d()->get_space());
				int point_count = PhysicsServer3D::get_singleton()->space_get_contact_count(find_world_3d()->get_space());
				RS::get_singleton()->multimesh_set_visible_instances(contact_3d_debug_multimesh, point_count);
				for (int i = 0; i < point_count; i++) {
					Transform3D point_transform;
					point_transform.origin = points[i];
					RS::get_singleton()->multimesh_instance_set_transform(contact_3d_debug_multimesh, i, point_transform);
				}
			}
#endif 
		} break;
		case NOTIFICATION_VP_MOUSE_ENTER: {