title: Type 'string' | null is not assignable to type 'string'
body: <p>I am using angular-jwt to to check whether token is expire or not and while using i am getting this error</p>
  <blockquote>
  <p>Type 'string' | null is not assignable to type 'string'</p>
  </blockquote>
  <pre><code>helper = new JwtHelperService();
  
  var token: string = localStorage.getItem('token');
  this.helper.isTokenExpired(token); // this line contains the error
  </code></pre>
  <blockquote>
  <p>Error</p>
  </blockquote>
  <p><a href="https://i.stack.imgur.com/LtNZT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LtNZT.png" alt="enter image description here" /></a></p>
  <p><a href="https://i.stack.imgur.com/cXZoP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cXZoP.png" alt="enter image description here" /></a></p>