### 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)
// Erase itself from viewport textures.
	for (ViewportTexture *E : viewport_textures) {
		E->vp = nullptr;
	}
	ERR_FAIL_NULL(RenderingServer::get_singleton());
	RenderingServer::get_singleton()->free(viewport);
}
//
void SubViewport::set_size(const Size2i &p_size) {
	_internal_set_size(p_size);
}
void SubViewport::set_size_force(const Size2i &p_size) {
	// Use only for setting the size from the parent SubViewportContainer with enabled stretch mode.
	// Don't expose function to scripting.
	_internal_set_size(p_size, true);
}
void SubViewport::_internal_set_size(const Size2i &p_size, bool p_force) {
	SubViewportContainer *c = Object::cast_to<SubViewportContainer>(get_parent());
	if (!p_force && c && c->is_stretch_enabled()) {
#ifdef DEBUG_ENABLED
		WARN_PRINT("Can't change the size of a `SubViewport` with a `SubViewportContainer` parent that has `stretch` enabled. Set `SubViewportContainer.stretch` to `false` to allow changing the size manually.");
#endif // DEBUG_ENABLED
		return;
	}
	_set_size(p_size, _get_size_2d_override(), Rect2i(), true);
	if (c) {
		c->update_minimum_size();
	}
}
Size2i SubViewport::get_size() const {