title: 'hidden' does not work when combining Angular with Semantic UI
body: <p>I use angular 4 in combination with Semantic UI. The following code works, if I use other classes:</p>
  
  <pre><code>&lt;form class="ui form" #transactionForm="ngForm" (ngSubmit)="onSubmit()"&gt;
    &lt;h3 class="ui header"&gt;Add a Transaction&lt;/h3&gt;
  
    &lt;div class="ui form error"&gt;
      &lt;div class="field"&gt;
        &lt;label&gt;Title&lt;/label&gt;
        &lt;input type="text"
               class="form-control"
               id="title"
               name="title"
               #newtitle="ngModel"
               placeholder="Title"
               [(ngModel)]="title"
               required&gt;{{newtitle.className}}
      &lt;/div&gt;{{newtitle.valid}}
      &lt;div [hidden]="newtitle.valid || newtitle.pristine" class="ui error message" #joh&gt;
        &lt;div class="header"&gt;Action Forbidden&lt;/div&gt;
        &lt;p&gt;This is the error message.&lt;/p&gt;
      &lt;/div&gt;
      {{joh.hidden}}
    &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>However: With how it is now, the <code>ui error message</code> won't get hidden. Can anyone explain why?</p>
  
  <p>I put some verifications to see if the attributes really are correct. It seems to be a semantic UI problem.</p>
  
  <p>The error-message should only show if the title field is empty. But it never hides.</p>