fix plugin translations

flaskbb/utils/translations.py

@@ -65,8 +65,7 @@ class FlaskBBDomain(Domain):
                     domain="messages"
                 )
 
-                if not isinstance(plugin_translation,
-                                  babel.support.NullTranslations):
+                if not type(plugin_translation) == babel.support.NullTranslations:
                     translations.add(plugin_translation)
 
             self.cache[str(locale)] = translations