Can't set a form through a directive

I'm trying to set a form through a directive. It seems that if I try to set it, its value is undefined.

Component

export class EditSalesComponent extends SalesDialogDirective implements OnInit {

    ngOnInit(): void {
        this.setForm(this.SalesFormGroup);
        this.SalesFormGroup.setHelper(this.helper);
      }

}

Directive

@ViewChild(HelperComponent) public helper: HelperComponent;
@ViewChild('SalesForm') public SalesFormGroup:SalesFormComponent;

public salesForm: SalesFormComponent;

public setForm = (form: SalesFormComponent): void => {
    this.salesForm = form;
  };

The error I'm getting is that salesForm isn't defined. Should I use a different lifecycle hook?