title: How to add "select all" functionality to ng-select in Angular 5
body: <p>I found an examle which can do "select all":
  <a href="https://ng-select.github.io/ng-select#/multiselect-checkbox" rel="noreferrer">https://ng-select.github.io/ng-select#/multiselect-checkbox</a></p>
  
  <p>But, I get an error: <code>Cannot read property 'selected' of undefined.</code>  I am wondering why I got this error, and how to implement "select all" using ng-select in Angular 5. </p>
  
  <p>Thank you</p>