title: Set formvalidator on FormArray for required and minlength
body: <p>I am looking to set a minlength validator on a formArray element. </p>
  
  <p>Here's how I created the form.</p>
  
  <pre><code>this.fb.group({
              FieldName: ['', [Validators.required, Validators.pattern(/^\S*/)]],
              Treatment: '',
              Properties: this.fb.group(
                  {
                      contentType: this.fb.array([]),
                      multipleItems: false,
                      keyType: this.fb.group({
                          'Type': '',
                          'Prefix': '',
                          'Suffix': ''
                      }),
                  }
              )
          });
  </code></pre>
  
  <p>This is the code I am trying. It doesn't get applied, and the form totally ignores this and becomes valid. </p>
  
  <pre><code>this.fieldForm.get('Properties.contentType').setValidators([Validators.required, Validators.minLength(1)]);
  </code></pre>