title: ng-select with form control required validation angular 7
body: <p>I'm using <code>ng-select</code> for dropdown search but I'm unable to get validation if not selecting anything from dropdown. 
  I return like bellow:</p>
  
  <pre><code>&lt;div class="form-group"&gt;
     &lt;ng-select [items]="productData" [searchable]="true" bindLabel="productName"
     [formControl]="prodCode"
     [ngClass]="{ 'is-invalid': submitted &amp;&amp; f.prodCode.errors }"
     placeholder="Select Product" required&gt;  
     &lt;/ng-select&gt;
     &lt;div *ngIf="submitted &amp;&amp; f.prodCode.errors" class="invalid-feedback"&gt;
        &lt;div *ngIf="f.prodCode.errors.required"&gt;Product Code is required&lt;/div&gt;
     &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>in Ts File </p>
  
  <pre><code>this.productForm = this.fb.group({
      prodCode: new FormControl(null, Validators.required)
  });
  get f() {
      return this.productForm.controls;
  }
  this.submitted = true;
  if (this.productForm.invalid) {
      return;
  }
  </code></pre>
  
  <p>So kindly requesting you please let me know where is my mistake....</p>