(fix) channel screen show alert twice if view permission is removed

src/channel/ChannelScreen.js

@@ -259,7 +259,7 @@ class ChannelScreen extends Component {
  * this is needed because in some cases the channel is shown using the owner of an activity
  * while we refresh the channel's data from the server
  */
- this.checkCanView(store.channel);
+ if (!this.checkCanView(store.channel)) return null;
 
  const feed = store.feedStore;
  const channel = store.channel;