### 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)
#include "camera_2d.h"
#include "core/config/project_settings.h"
#include "scene/main/window.h"
void Camera2D::_update_scroll() {
	if (!is_inside_tree()) {
		return;
	}
	if (Engine::get_singleton()->is_editor_hint()) {
		queue_redraw();
		
		if (get_viewport() == get_tree()->get_edited_scene_root()->get_viewport()) {
			return;
		}
	}
	if (!viewport) {
		return;
	}
	if (is_current()) {
		ERR_FAIL_COND(custom_viewport && !ObjectDB::get_instance(custom_viewport_id));
		Transform2D xform = get_camera_transform();
		viewport->set_canvas_transform(xform);
		Size2 screen_size = _get_camera_screen_size();
		Point2 screen_offset = (anchor_mode == ANCHOR_MODE_DRAG_CENTER ? (screen_size * 0.5) : Point2());
		get_tree()->call_group(group_name, "_camera_moved", xform, screen_offset);
	};
}
void Camera2D::_update_process_callback() {