Error in the log when sending a mail through the Mail API

When sending a mail with the mail API, the mail is successful, but the logs report an error in the mail sender thread.

To reproduce:

1. Configure mail sending
2. Try a simple example from the documentation http://extensions.xwiki.org/xwiki/bin/view/Extension/Mail+Sender+API#HExample1:Sendasimpletextemail
3. Observe the logs:

   2016-07-05 12:28:01,067 [Mail Sender Thread] ERROR o.x.m.i.t.SendMailRunnable     - Unexpected error in the Mail Sender Thread 
   java.lang.IllegalArgumentException: An Entity Reference name cannot be null or empty
           at org.xwiki.model.reference.EntityReference.setName(EntityReference.java:186)
           at org.xwiki.model.reference.EntityReference.<init>(EntityReference.java:171)
           at org.xwiki.model.reference.EntityReference.<init>(EntityReference.java:142)
           at org.xwiki.model.reference.WikiReference.<init>(WikiReference.java:61)
           at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.getCurrentWikiReference(AbstractDocumentConfigurationSource.java:185)
           at org.xwiki.mail.internal.configuration.SendMailConfigClassDocumentConfigurationSource.getDocumentReference(SendMailConfigClassDocumentConfigurationSource.java:68)
           at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.getCacheKeyPrefix(AbstractDocumentConfigurationSource.java:122)
           at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.getPropertyValue(AbstractDocumentConfigurationSource.java:306)
           at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.getProperty(AbstractDocumentConfigurationSource.java:301)
           at org.xwiki.mail.internal.configuration.DefaultMailSenderConfiguration.getSendWaitTime(DefaultMailSenderConfiguration.java:313)
           at org.xwiki.mail.internal.thread.SendMailRunnable.waitSendWaitTime(SendMailRunnable.java:177)
           at org.xwiki.mail.internal.thread.SendMailRunnable.runInternal(SendMailRunnable.java:108)
           at org.xwiki.mail.internal.thread.SendMailRunnable.run(SendMailRunnable.java:74)
           at java.lang.Thread.run(Thread.java:745)