Address null-safety identified by TSC compiler v6

There are no actual functional changes. This just adds many additional
annotations and function which ensure type safety and null safety.

radicale/web/internal_data/js/scenes/LoadingScene.js

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+import { get_element_by_id } from "../utils/misc.js";
 import { Scene } from "./scene_manager.js";
 
 /**
@@ -26,7 +27,7 @@ import { Scene } from "./scene_manager.js";
  */
 export class LoadingScene {
     constructor() {
-        this.html_scene = document.getElementById("loadingscene");
+        this.html_scene = get_element_by_id("loadingscene");
     }
     show() {
         this.html_scene.classList.remove("hidden");