correct handling when adding new cluster numbers

src/message/simple_edits.py

@@ -111,7 +111,5 @@ class ExampleClusterEdit(DataChgClickMessage):
 class ExampleClusterAdd(DataChgClickMessage):
     def update_model(self, model):
         example = self.get_arg(0, Example)
-        
-        max_example_cluster_num = ExampleClusters.first_empty_cluster()
-        example.set_cluster(max_example_cluster_num + 1)
+        example.set_cluster(ExampleClusters.first_empty_cluster())