title: Set checkbox validation if checkbox unchecked
body: <p>In my Angular 6 application's register form, I have a checkbox which is already checked. I need to check whether the user has unchecked the checkbox and if it is so I want to display an error message.</p>
  
  <p>This is my checkbox in .html file,</p>
  
  <pre><code> &lt;div class="form-group"&gt;
         &lt;div class="round"&gt;
              &lt;input
                    type="checkbox"
                    id="checkbox33"
                    formControlName="agree"&gt;
               &lt;label for="checkbox33"&gt;&lt;/label&gt;
          &lt;/div&gt;
          &lt;small class="text-muted"&gt;Agree to our &lt;a href="#" class="link"&gt;Terms
                    Of Use&lt;/a&gt; and &lt;a href="#" class="link"&gt;Privacy Policy&lt;/a&gt;.
                  &lt;/small&gt;
          &lt;span *ngIf="!worldWideRegisterForm.get('agree').valid" class="text-danger"&gt;You should agree!&lt;/span&gt;
  
    &lt;/div&gt;
  </code></pre>
  
  <p>This is my ngOnInit() in .ts file</p>
  
  <pre><code>ngOnInit() {
      this.worldWideRegisterForm = new FormGroup({
        'userName': new FormGroup({
          firstName: new FormControl(null, Validators.required),
          lastName: new FormControl(null, Validators.required)
        }),
        'email': new FormControl(null, [Validators.required, Validators.email]),
        'phone': new FormControl(null, Validators.required),
        'address': new FormGroup({
          line1: new FormControl(null, Validators.required),
          line2: new FormControl(null),
        }),
        'area': new FormGroup({
          'province': new FormControl(null, Validators.required),
          'city': new FormControl(null, Validators.required),
          'postalCode': new FormControl(null, Validators.required),
        }),
        'password': new FormControl(null, [Validators.required, Validators.minLength(6)]),
        'agree': new FormControl(true, Validators.required)
      });
    }
  </code></pre>