Where is Null pointer?

I have a Null Pointer

I check all my variable at my code

I can't find a possible Null pointer
I know It come from ExternalMessage Class

It said like below

msg = new ExternalMessage();
msg.sendMessage(sender,receiver,subject,content)

Finally I got the idea
This sendMessage have addMessage method call inside.

The problem is the Exception wrapper

It hide the real problem