Check if user logged in before making setActiveChannel request (#4097)

webapp/components/post_view/post_view_cache.jsx

@@ -4,6 +4,7 @@
 import PostViewController from './post_view_controller.jsx';
 
 import ChannelStore from 'stores/channel_store.jsx';
+import UserStore from 'stores/user_store.jsx';
 import * as AsyncClient from 'utils/async_client.jsx';
 
 import React from 'react';
@@ -29,7 +30,9 @@ export default class PostViewCache extends React.Component {
     }
 
     componentWillUnmount() {
-        AsyncClient.setActiveChannel('');
+        if (UserStore.getCurrentUser()) {
+            AsyncClient.setActiveChannel('');
+        }
         ChannelStore.removeChangeListener(this.onChannelChange);
     }