Fix a crashing issue with autocomplete building

Signed-off-by: Mario Danic <mario@lovelyhq.com>

app/src/main/java/com/nextcloud/talk/controllers/ChatController.java

@@ -459,13 +459,15 @@ public class ChatController extends BaseController implements MessagesListAdapte
         AutocompletePresenter<Mention> presenter = new MentionAutocompletePresenter(getApplicationContext(), roomToken);
         AutocompleteCallback<Mention> callback = new MentionAutocompleteCallback();
 
-        mentionAutocomplete = Autocomplete.<Mention>on(messageInput)
-                .with(elevation)
-                .with(backgroundDrawable)
-                .with(new CharPolicy('@'))
-                .with(presenter)
-                .with(callback)
-                .build();
+        if (mentionAutocomplete != null) {
+            mentionAutocomplete = Autocomplete.<Mention>on(messageInput)
+                    .with(elevation)
+                    .with(backgroundDrawable)
+                    .with(new CharPolicy('@'))
+                    .with(presenter)
+                    .with(callback)
+                    .build();
+        }
     }
 
     @Override