ngx-select does not show invalid style when used with Validators.required

[joeveiga]

Screenshot

Steps to reproduce

Add this snippet to the SelectPageComponent demo page:

  // ...
  selectControl = new FormControl(['breach'], Validators.required);
  // ...

And change the template to:

  <!-- ... -->
  <h4>Basic</h4>
  <ngx-select [filterable]="false" [label]="'Attack Type'" [formControl]="selectControl" data-cy="attackType">
    <ngx-select-option name="Breach" value="breach"></ngx-select-option>
    <ngx-select-option name="DDOS" value="ddos"></ngx-select-option>
    <ngx-select-option name="Physical" value="physical"></ngx-select-option>
  </ngx-select>

  Is valid: {{ selectControl.valid }}
  <!-- ... -->