Clear mention autocomplete if there is no regex match (#5085)

webapp/components/suggestion/at_mention_provider.jsx

@@ -6,6 +6,7 @@ import Provider from './provider.jsx';
 
 import ChannelStore from 'stores/channel_store.jsx';
 import UserStore from 'stores/user_store.jsx';
+import SuggestionStore from 'stores/suggestion_store.jsx';
 
 import {autocompleteUsersInChannel} from 'actions/user_actions.jsx';
 
@@ -161,6 +162,8 @@ export default class AtMentionProvider extends Provider {
                     });
                 }
             );
+        } else {
+            SuggestionStore.clearSuggestions(suggestionId);
         }
     }
 }