### 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)
canvas_group_name = "__cameras_c" + itos(canvas.get_id());
			add_to_group(group_name);
			add_to_group(canvas_group_name);
			if (enabled && !viewport->get_camera_2d()) {
				make_current();
			}
			_update_process_callback();
			first = true;
			_update_scroll();
		} break;
		case NOTIFICATION_EXIT_TREE: {
			if (is_current()) {
				clear_current();
			}
			remove_from_group(group_name);
			remove_from_group(canvas_group_name);
			viewport = nullptr;
		} break;
#ifdef TOOLS_ENABLED
		case NOTIFICATION_DRAW: {
			if (!is_inside_tree() || !Engine::get_singleton()->is_editor_hint()) {
				break;
			}
			if (screen_drawing_enabled) {
				Color area_axis_color(1, 0.4, 1, 0.63);
				real_t area_axis_width = 1;
				if (is_current()) {
					area_axis_width = 3;
				}
				Transform2D inv_camera_transform = get_camera_transform().affine_inverse();