Type 'null' is not assignable to type 'ValidationErrors'

So, my issue is I am trying to build a custom validator on Angular 15, and I get an error message that tells this:

Type 'Observable<{ titleAlreadyExists: boolean; } | null>' is not assignable to type 'Observable'. Type '{ titleAlreadyExists: boolean; } | null' is not assignable to type 'ValidationErrors'. Type 'null' is not assignable to type 'ValidationErrors'."

This is my validator:

alreadyExistingTitle(alreadyExistingTitles: String[]): AsyncValidatorFn {
  return (control: AbstractControl): Observable<ValidationErrors> => {
    return of(alreadyExistingTitles.includes(control.value))
      .pipe(
        map((result: boolean) =>
          result ? { titleAlreadyExists: true } : null
        )
      );
};

Modify the return type from Observable<ValidationErrors> to Observable<ValidationErrors | null>.

alreadyExistingTitle(alreadyExistingTitles: String[]): AsyncValidatorFn {
  return (control: AbstractControl): Observable<ValidationErrors | null> => {
    return of(alreadyExistingTitles.includes(control.value)).pipe(
      map((result: boolean) => (result ? { titleAlreadyExists: true } : null))
    );
  };
}