Angular changeable form according to inputs - html

Question

I'm using a reactive form. I need to add/remove an input that appears in it according to some other input. Here's a simplified scenario of the issue:
Asking the user to select an option from a list. If their desired option is not there, there is an open input where they can write. If they do choose an option from the select, the input must disappear. If they do not select an option, the input must be there and it must be required.
Here's the code I made which 1) doesn't work 2) feels like it's fairly ugly and could be made in some other way.
Template:
<form [formGroup]="whateverForm" (ngSubmit)="onSubmit()">
Choose an option:
<select
formControlName="option"
(change)="verifySelection($event)">
<option value=''>None</option>
<option value='a'>Something A</option>
<option value='b'>Something B</option>
</select>
<br>
<div *ngIf="!optionSelected">
None of the above? Specify:
<input type="text" formControlName="aditional">
</div>
<br>
<br>
Form current status: {‌{formStatus}}
</form>
Code:
export class AppComponent {
whateverForm: FormGroup;
formStatus: string;
optionSelected = false;
ngOnInit() {
this.whateverForm = new FormGroup({
'option': new FormControl(null, [Validators.required]),
'aditional': new FormControl(null, [Validators.required])
});
this.whateverForm.statusChanges.subscribe(
(status) => {
this.formStatus = status;
}
);
}
verifySelection(event: any) {
if (event.target.value !== '') {
this.optionSelected = true;
this.whateverForm.get('aditional').clearValidators();
this.whateverForm.get('option').setValidators(
[Validators.required]);
} else {
this.optionSelected = false;
this.whateverForm.get('option').clearValidators();
this.whateverForm.get('aditional').setValidators(
[Validators.required]);
}
}
}

Answer 1

Instead of using an event, I used an observable in one of the fields. The exact solution to the problem I proposed is here.
And I solved it using what I found here (they are basically the same thing, but I included mine for completion).