title: Type 'Observable<string | null>' is not assignable to type 'string'
body: <p>I have code:</p>
  <pre><code>id$!: Observable&lt;string | null&gt;;
   characterDetail$!: Observable&lt;CharacterData | undefined&gt;;   //??????
  
   constructor(
     private router: ActivatedRoute,
     private store$: Store
   ) {} 
  
   ngOnInit(): void {
     this.id$ = of(this.router.snapshot.paramMap.get('id'));
     this.getCharacterDetail();
     this.characterDetail$ = this.store$.select(charDetailsSelector);
   }
  
   getCharacterDetail() { 
     if (!this.id$) {
       return;
     }
     this.store$.dispatch(getCharDetailsData({id: this.id$ }));
  
   }
  </code></pre>
  <p>and error in this part:
  <a href="https://i.stack.imgur.com/tWxgv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tWxgv.png" alt="enter image description here" /></a></p>