title: Reactive form using multiple selection on new option selection error
body: <p>Is there an example on <code>how to implement multiple selection and reactive form ?</code></p>
  
  <p>I'm trying to set the selected items (retrieved from the database) on the <code>multiselect-dropdown</code>, it will update the items displayed (the view), but it throws following error:</p>
  
  <pre><code>core.js:12501 ERROR TypeError: control.registerOnChange is not a function
      at setUpModelChangePipeline (forms.js:2064)
      at setUpControl (forms.js:2007)
      atFormGroupDirective.push../node_modules/@angular/forms/fesm5/forms.js.FormGroupDirective.addControl 
      (forms.js:4969)
      at FormControlName.push../node_modules/@angular/forms/fesm5/forms.js.FormControlName._setUpControl 
      (forms.js:5572)
      at FormControlName.push../node_modules/@angular/forms/fesm5/forms.js.FormControlName.ngOnChanges 
      (forms.js:5491)
      at checkAndUpdateDirectiveInline (core.js:18533)
      at checkAndUpdateNodeInline (core.js:19801)
      at checkAndUpdateNode (core.js:19763)
      at prodCheckAndUpdateNode (core.js:20304)
      at Object.eval [as updateDirectives] (AddAppointmentsComponent.ngfactory.js:646)
  </code></pre>
  
  <p>I was trying to select new option getting error.</p>
  
  <pre><code>core.js:12501 ERROR Error: Cannot find form control at index 3
      at FormArray.push../node_modules/@angular/forms/fesm5/forms.js.FormArray._throwIfControlMissing (forms.js:4001)
      at forms.js:3870
      at Array.forEach (&lt;anonymous&gt;)
      at FormArray.push../node_modules/@angular/forms/fesm5/forms.js.FormArray.setValue (forms.js:3869)
      at updateControl (forms.js:2059)
      at MultiSelectComponent.onChangeCallback (forms.js:2044)
      at MultiSelectComponent.push../node_modules/ng-multiselect-dropdown/fesm5/ng-multiselect- 
      dropdown.js.MultiSelectComponent.addSelected (ng-multiselect-dropdown.js:348)
      at MultiSelectComponent.push../node_modules/ng-multiselect-dropdown/fesm5/ng-multiselect- 
      dropdown.js.MultiSelectComponent.onItemClick (ng-multiselect-dropdown.js:160)
      at Object.eval [as handleEvent] (MultiSelectComponent.ngfactory.js:135)
      at Object.handleEvent (core.js:19545)
  </code></pre>
  
  <p><strong>data</strong></p>
  
  <pre><code>"questionnaireData":[{"questionnaireId":"Xvqo5atpw",
  
                        "timeRepeat":[    
                                      {"item_id":1,"item_text":"01:00"},
                                      {"item_id":2,"item_text":"02:00"},
                                      {"item_id":2,"item_text":"02:00"}
                                     ]    
                      }],
  </code></pre>
  
  <p>In the component <code>formArray</code> assigning to <code>formBuilder.array(ap.timeRepeat)</code>.</p>
  
  <p><strong>component.ts</strong></p>
  
  <pre><code>this.addAppointmentForm.setControl('questionnaireData', 
  this.setExistingQuestionnairesDetails(this.AppointmentList.questionnaireData));
  
  setExistingQuestionnairesDetails(questionDataArray: IQuestionnaireData[]): FormArray {
      const formArray = new FormArray([]);
      console.log(questionDataArray);
      questionDataArray.forEach(ap=&gt;{
  
        formArray.push(this.formBuilder.group({
          questionnaireId: ap.questionnaireId,
          timeRepeat: this.formBuilder.array(ap.timeRepeat),
  
        }))
      })
      return formArray;
    }
  
  </code></pre>
  
  <p><strong>compnent.html</strong></p>
  
  <pre><code>&lt;ng-multiselect-dropdown formControlName="timeRepeat" [data]="dropdownList"
    [(ngModel)]="timeRepeat" [settings]="dropdownSettings" (onSelect)="onItemSelect($event)" 
    (onSelectAll)="onSelectAll($event)" (onDeSelect)="onDeSelect($event)"&gt;
   &lt;/ng-multiselect-dropdown&gt;
  </code></pre>
  
  <p>Please let me know how I can get around this error or some better way to get things done.</p>