I have seen null ! many times in source code of Angular. What's the meaning of the following ! mark?

examples:

• https://github.com/angular/angular/blob/master/packages/router/src/router.ts#L221

  private navigations = new BehaviorSubject<NavigationParams>(null !);
  

• https://github.com/angular/angular/blob/master/packages/router/src/router.ts#L357

  this.locationSubscription = null !;