title: How to control opening multiple dialogs
body: <p>I have created modal dialog in angular 2 using angular Material UI.
  App has two buttons, when we click any button the dialog should open. I am able to open modal dialog on button click but multiple dialog are opened when we continuously click on button.
  How we can do opening only one dialog and restricting the app to open another dialog if one is already present.</p>
  
  <p>Below is the APP link</p>
  
  <p><a href="https://stackblitz.com/edit/angular-y8l9fz?file=app/comment-dialog.html" rel="noreferrer">Angular 2 App</a></p>