title: ERROR Error: mat-menu-trigger: must pass in an mat-menu instance
body: <p>I am trying to create a menu top of my page using mat-menu and matMenuTriggerFor, 
  I found an example in material website(<a href="https://material.angular.io/components/menu/examples" rel="noreferrer">https://material.angular.io/components/menu/examples</a>) and tried to implement it, everything works fine buut I am getting following error.</p>
  
  <pre><code>ERROR Error: mat-menu-trigger: must pass in an mat-menu instance.
  Example:
    &lt;mat-menu #menu="matMenu"&gt;&lt;/mat-menu&gt;
    &lt;button [matMenuTriggerFor]="menu"&gt;&lt;/button&gt;"
  </code></pre>
  
  <p>in the console.
  I don't know the reason. Anyone know the solution? </p>