title: Angular reactive form invalid after pressing back button
body: <p>I got an Angular Reactive Form working well except when I fill all the required fields and I press Chrome's back button (after submitting it or not) and go back to the form page again. My form is still filled but I can't submit the form again. I guess it's because the form validation is invalid, I have to re fill every field in order to press the submit button again.</p>
  
  <p>How can I force the re validation of the form so I don't have to erase and type everything again?</p>
  
  <pre><code>  profileForm = this.fb.group({
      checkbox: [false],
      checkbox_cgv: [false, Validators.requiredTrue],
      firstName: ['', Validators.required],
      lastName: ['', Validators.required],
      email:['', [Validators.required, Validators.email]],
      phoneNumber:['', Validators.required],
      billing: this.fb.group({
        address1: ['', Validators.required],
        address2: [''],
        city: ['', Validators.required],
        zip: ['', Validators.required]
      }),
      shipping: this.fb.group({
        firstName: ['', Validators.required],
        lastName: ['', Validators.required],
        address1: ['', Validators.required],
        address2: [''],
        city: ['', Validators.required],
        zip: ['', Validators.required]
      }),
    });
  </code></pre>
  
  <p>Part of the html file:</p>
  
  <pre><code>&lt;form [formGroup]="profileForm" (ngSubmit)="onSubmit()"&gt;
      &lt;div class="row"&gt;
        &lt;div class="col"&gt;
          &lt;p class="form-row form-left"&gt;
            &lt;span class="input-wrapper"&gt;
              &lt;input type="text" class="form-control" formControlName="firstName" required&gt;
            &lt;/span&gt;
          &lt;/p&gt;
          &lt;p class="form-row form-right"&gt;
            &lt;span class="input-wrapper"&gt;
              &lt;input type="text" class="form-control" formControlName="lastName" required&gt;
            &lt;/span&gt;
          &lt;/p&gt;
  ...
  
  &lt;button type="submit" class="btn btn-primary order-btn" [disabled]="!profileForm.valid"&gt;Order&lt;/button&gt;
  </code></pre>