If there is an exception while sending mail which has no message (NPE) another exception is thrown hiding the real one.

When an NPE or other exception with no message is thrown, you can't see the stack trace because it is supposed to be caught and the message put in the context but if there is no message, the catch block throws an NPE which hides the stack trace of the original exception.
Failure trying to put a non-existant message into the context:

Caused by: java.lang.NullPointerException
	at java.util.Hashtable.put(Hashtable.java:394)
	at com.xpn.xwiki.plugin.mailsender.MailSenderPluginApi.sendMail(MailSenderPluginApi.java:195)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)