title: control.setParent is not a function when dymanically creating formGroup
body: <p>I'm using Angular5, and I have a list of fields, each has a name and FormControl. I try to dynamically add the controls to the group, using this code, but I got an error.</p>
  
  <pre><code>const formControlFields = [
    {name: 'field1',control: [null, []] as FormControl},
    {name: 'field2',control: [null, []] as FormControl}
  ];
  
  const formGroup: FormGroup = new FormGroup({});
  formControlFields.forEach( f =&gt;  formGroup.addControl(f.name,f.control));
  this.form = new FormGroup({groups:formGroup});
  </code></pre>
  
  <p>This is the error I get:</p>
  
  <blockquote>
    <p>ERROR TypeError:
    control.setParent is not a function</p>
  
  <pre><code>at FormGroup.registerControl (forms.js:4352)
  at FormGroup.addControl (forms.js:4372)
  at eval (trade-search.component.ts:142)
  </code></pre>
  </blockquote>