fix possible NullPointerException

scm-webapp/src/main/java/sonia/scm/api/rest/resources/RepositoryResource.java

@@ -415,12 +415,15 @@ public class RepositoryResource
         {
           ChangesetPreProcessor cpp = factory.createPreProcessor(repository);
 
+          if (cpp != null)
+          {
             for (Changeset c : changesets.getChangesets())
             {
               cpp.process(c);
             }
           }
         }
+      }
       else if (logger.isWarnEnabled())
       {
         logger.warn("could not find repository {}", id);