### 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 (use_xr != p_use_xr) {
		use_xr = p_use_xr;
		RS::get_singleton()->viewport_set_use_xr(viewport, use_xr);
		if (!use_xr) {
			
			if (size_allocated) {
				RS::get_singleton()->viewport_set_size(viewport, size.width, size.height);
			} else {
				RS::get_singleton()->viewport_set_size(viewport, 0, 0);
			}
		}
	}
}
bool Viewport::is_using_xr() {