Reactive Form value from Firebase

Hello everyone, I have a probem with a value of validator in a reactive form. In particular I have this 2 function below that check if a username in a realtime database of firebase exists. If it doesn’t exist then the function return a Promise validUsername:

export class UsernameValidator {
  static async getSnapshot(fc: FormControl){
    let isPresent:boolean = false;
    await firebase.database().ref().child("users").orderByChild("username")
    .equalTo(fc.value)
    .once("value", snapshot => {          
    }).then((data)=> {
      if(data.exists())
        isPresent = true;
      else
        isPresent = false;
    });
    console.log(isPresent);
    return isPresent; 
  }

  static async validUsername(fc: FormControl){
    try{
      let present:boolean =await UsernameValidator.getSnapshot(fc)
      if(present==true)
      return  (null);         
   else{
      return ({validUsername: true}); 
    } 
      }catch(e){
        console.log(e)
      }         
  }

Then , I have I class in which I define a FormGroup and validator:

constructor(private route: ActivatedRoute, private router: Router, 
              public pfService: ProfileService, public fb: FormBuilder,
              public authService: AuthenticationService) 
  {
    this.id = this.authService.userData.uid;
    //Underscore and dot can't be next to each other (e.g user_.name).
    //Underscore or dot can't be used multiple times in a row (e.g user__name / user..name).
    this.validPattern = "^(?=.{6,20}$)(?!.*[_.]{2})[a-z0-9._]+$"; 
    this.validPatternName = "^[a-z]{3,10}$";
    this.userForm = fb.group({
      txtUsername:  ["",[Validators.required,Validators.pattern(this.validPattern),
                                                  UsernameValidator.validUsername]],
      txtName:     ["",[Validators.required,Validators.pattern(this.validPatternName)]],
    });
    this.userForm .valueChanges.subscribe(()=> {
      console.log(this.userForm.getError('validUsername'))
      })
  };

The problem is that validUsername , from the console, is always null regardless of the value of isPresent. How can I fix this?

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