Merge pull request #2595 from hmhealey/plt2480

PLT-2480 Fixed errors when joining a channel that doesn't exist yet

webapp/components/posts_view.jsx

@@ -374,6 +374,7 @@ export default class PostsView extends React.Component {
     componentWillUnmount() {
         window.removeEventListener('resize', this.handleResize);
         this.scrollStopAction.cancel();
+        PreferenceStore.removeChangeListener(this.updateState);
     }
     componentDidUpdate() {
         if (this.props.postList != null) {