core.js:5980 error error: ng0100: expressionchangedafterithasbeencheckederror: expression has changed after it was checked. previous value for 'mat-form-field-should-float': 'true'. current value: 'false'. comes from dropdown code example

constructor(private ref: ChangeDetectorRef){}

  ngAfterContentChecked() {
    this.ref.detectChanges();
  }