title: Angular 11 - Type '(controlArray: FormArray) => ValidationErrors | null' is not assignable to type 'ValidatorFn'
body: <p>I try to implement a validator in Angular 11 but i get the compliation error:
  <code>Type '(controlArray: FormArray) =&gt; ValidationErrors | null' is not assignable to type 'ValidatorFn'.</code> Any ideas how to fix this error?</p>
  <p><strong>Validator-class:</strong></p>
  <pre><code>import { FormArray, ValidationErrors } from &quot;@angular/forms&quot;;
  
  export class BookValidators {
      static atLeastOneAuthor(controlArray: FormArray): ValidationErrors | null {
          if(controlArray.controls.some(el =&gt; el.value)){
              return null;
          }else{
              return{
                  atLeastOneAuthor: {
                      valid: false
                  }
              }
          }
      }
  }
  </code></pre>
  <p><strong>The calling class</strong></p>
  <pre><code>constructor(private fb: FormBuilder){}
  
  private buildAuthorsArray(values: string[]): FormArray{
      // BookValidators.atLeastOneAuthor is now underlined with the error.
      return this.fb.array(values, BookValidators.atLeastOneAuthor);
  }
  </code></pre>