title: postrender event from Vectorlayer does not work with openlayers 6
body: <p>We are using openlayer 6.3.1 und want to use "postrender" event from Layer Api, but it does not work right.</p>
  
  <p>The code is like this:</p>
  
  <pre><code>vectorLayer.on("postrender", this.moveFeature);
  ......
  moveFeature: function (event) {
      console.log(event);
  ...
  </code></pre>
  
  <p><strong>The console log message will not show.</strong></p>
  
  <p>But if i change the code to:</p>
  
  <pre><code>vectorLayer.on("postrender", console.log("test"));
  </code></pre>
  
  <p><strong>the log message "test" will be shown in console,</strong> </p>
  
  <p>If i change the code to:</p>
  
  <pre><code>vectorLayer.on("postrender", function(evt) {
      console.log(evt);
  });
  </code></pre>
  
  <p>there is also <strong>no console messages</strong>, what could be the problem? Our project is backbone basis, and we use many backbone event, i dont know if it make some troubles. </p>