Merge pull request #496 from swagger-api/ticket-495

refs #495 - fix NPE

Author: frantuma

src/main/java/io/swagger/codegen/v3/generators/java/JavaClientCodegen.java

@@ -521,7 +521,8 @@ protected List<Map<String, Object>> modelInheritanceSupportInGson(List<?> allMod
             }
         }
         List<Map<String, Object>> parentsList = new ArrayList<>();
-        for (CodegenModel parentModel : byParent.keySet()) {
+        for (Map.Entry<CodegenModel, List<CodegenModel>> parentModelEntry : byParent.entrySet()) {
+            CodegenModel parentModel = parentModelEntry.getKey();
             List<Map<String, Object>> childrenList = new ArrayList<>();
             Map<String, Object> parent = new HashMap<>();
             parent.put("classname", parentModel.classname);
@@ -540,6 +541,7 @@ protected List<Map<String, Object>> modelInheritanceSupportInGson(List<?> allMod
             }
             parentsList.add(parent);
         }
+
         return parentsList;
     }