ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'model: undefined'. Current value: 'model: null'. code example

Example: angular ERROR Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked.

constructor(private ref: ChangeDetectorRef){}

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