fixed npe

Author: HugoMario

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/CSharpClientCodegen.java

@@ -491,7 +491,7 @@ public Map<String, Object> postProcessOperations(Map<String, Object> objs) {
     @Override
     public String getSwaggerType(Property p) {
         String swaggerType = super.getSwaggerType(p);
-        if (p instanceof RefProperty) {
+        if (p instanceof RefProperty && this.swagger != null) {
             final Map<String, Model> allDefinitions = this.swagger.getDefinitions();
             final Model referencedModel = allDefinitions.get(swaggerType);
             if (referencedModel == null) {