title: Angular material stepper: Error: Cannot find control with name: 'formArray'
body: <p>I tried to follow up with the steps on how to setup angular material stepper here: <a href="https://material.angular.io/components/stepper/overview" rel="nofollow noreferrer">https://material.angular.io/components/stepper/overview</a></p>
  
  <p>I wanted a simple single form with multiple steps so I created this:</p>
  
  <pre><code>&lt;form [formGroup]="formGroup"&gt;
    &lt;mat-horizontal-stepper formArrayName="formArray" linear&gt;
      &lt;mat-step formGroupName="0" [stepControl]="formArray.get([0])"&gt;
        &lt;div&gt;
          &lt;button mat-button matStepperNext type="button"&gt;Next&lt;/button&gt;
        &lt;/div&gt;
      &lt;/mat-step&gt;
      &lt;mat-step formGroupName="1" [stepControl]="formArray.get([1])"&gt;
        &lt;div&gt;
          &lt;button mat-button matStepperPrevious type="button"&gt;Back&lt;/button&gt;
          &lt;button mat-button matStepperNext type="button"&gt;Next&lt;/button&gt;
        &lt;/div&gt;
      &lt;/mat-step&gt;
    &lt;/mat-horizontal-stepper&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>I get on my console: </p>
  
  <pre><code>ERROR Error: Cannot find control with name: 'formArray'
      at _throwError (forms.es5.js:1918)
      at setUpFormContainer (forms.es5.js:1891)
      at FormGroupDirective.webpackJsonp.../../../forms/@angular/forms.es5.js.FormGroupDirective.addFormArray (forms.es5.js:4849)
      at FormArrayName.webpackJsonp.../../../forms/@angular/forms.es5.js.FormArrayName.ngOnInit (forms.es5.js:5134)
      at checkAndUpdateDirectiveInline (core.es5.js:10856)
      at checkAndUpdateNodeInline (core.es5.js:12364)
      at checkAndUpdateNode (core.es5.js:12303)
      at debugCheckAndUpdateNode (core.es5.js:13167)
      at debugCheckDirectivesFn (core.es5.js:13108)
      at Object.eval [as updateDirectives] (MystepperComponent.htm
  </code></pre>
  
  <p>The full source code of my test is here: <strong><a href="https://github.com/avidanyum/steppercheck" rel="nofollow noreferrer">angular stepper test example on github</a></strong></p>
  
  <p>I really did my best to follow the documentation, but i don't understand what I need to do to fix it...</p>