title: Angular 4 Reactive Forms FormControl errors is null
body: <p>If I tab through the text inputs without entering anything, the error msg divs display indicating that the required validator has fired correctly.  However, if I type anything into one of the fields, the console immediately throws this error:</p>
  
  <pre><code>Cannot read property 'required' of null
  </code></pre>
  
  <p>Here is my component:</p>
  
  <pre><code>import { PasswordValidators } from './../validators/password.validators';
  import { Component, OnInit } from '@angular/core';
  import { FormBuilder, Validators, FormGroup, FormControl } from '@angular/forms';
  
  @Component({
    selector: 'app-changepassword-form',
    templateUrl: './changepassword-form.component.html',
    styleUrls: ['./changepassword-form.component.css']
  })
  export class ChangepasswordFormComponent {
  
    form;
  
    constructor(fb: FormBuilder) {
      this.form = fb.group({
        newPassword: ['', Validators.required],
        confirmPassword: ['', Validators.required]
      })
    }
  
    get newPassword() {
      return this.form.get('newPassword');
    }
  
    get confirmPassword() {
      return this.form.get('confirmPassword');
    }
  
  }
  </code></pre>
  
  <p>and HTML:</p>
  
  <pre><code>&lt;form [formGroup]="form"&gt;  
    &lt;div class="form-group"&gt;
      &lt;label for="newPassword"&gt;New Password&lt;/label&gt;
      &lt;input formControlName="newPassword" id="newPassword" type="text" class="form-control"&gt;
      &lt;div class="alert alert-danger" *ngIf="newPassword.touched &amp;&amp; newPassword.errors.required"&gt;
        Required
      &lt;/div&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label for="confirmPassword"&gt;Confirm Password&lt;/label&gt;
      &lt;input formControlName="confirmPassword" id="confirmPassword" type="text" class="form-control"&gt;
      &lt;div class="alert alert-danger" *ngIf="confirmPassword.touched &amp;&amp; confirmPassword.errors.required"&gt;
        Required
      &lt;/div&gt;
    &lt;/div&gt;
  
    &lt;p&gt;&lt;button class="btn btn-primary"&gt;Submit&lt;/button&gt;&lt;/p&gt;
  
  &lt;/form&gt;
  </code></pre>