Check result handler is not null

If caller passes a null result handler to the proxy methods, we need
to check it when getting the result. Otherwise we try to invoke a method
on a null object

Change-Id: Ic2f456c36cb542b782b6a6d0d4d607ccf0987496
diff --git a/tools/aidl/generate_java_rpc.cpp b/tools/aidl/generate_java_rpc.cpp
index 2b50b76..21ebb1f 100644
--- a/tools/aidl/generate_java_rpc.cpp
+++ b/tools/aidl/generate_java_rpc.cpp
@@ -784,7 +784,10 @@
     }
 
     // Call the callback method
-    dispatchMethod->statements->Add(realCall);
+    IfStatement* ifst = new IfStatement;
+        ifst->expression = new Comparison(new FieldVariable(THIS_VALUE, "callback"), "!=", NULL_VALUE);
+    dispatchMethod->statements->Add(ifst);
+    ifst->statements->Add(realCall);
 }
 
 static void