null check

Author: fehguy

modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultGenerator.java

@@ -259,7 +259,10 @@ private int getInheritanceDepth(Model model) {
 
                     private Model getParent(Model model) {
                         if (model instanceof ComposedModel) {
-                            return definitions.get(((ComposedModel) model).getParent().getReference());
+                            Model parent = ((ComposedModel) model).getParent();
+                            if(parent != null) {
+                                return definitions.get(parent.getReference());
+                            }
                         }
 
                         return null;