Error when looking for a previous mail status for message

1. Open a local clean instance of XE 8.2 with default DB.
2. Configure your email server.
3. Create a test page and add this code in Source mode (http://extensions.xwiki.org/xwiki/bin/view/Extension/Mail+Sender+API#HExample1:Sendasimpletextemail)

{{velocity}}
#set ($message = $services.mailsender.createMessage("localhost@xwiki.org", "john@doe.com", "subject"))
#set ($discard = $message.addPart("text/plain", "text content"))
#set ($mailResult = $services.mailsender.send([$message], 'database'))
## Check if the message was created properly and if we have permissions to send emails
#if ($services.mailsender.lastError)
  {{error}}$exceptiontool.getStackTrace($services.mailsender.lastError){{/error}}
#end
## Check if the mail we tried to send has failed to be sent
## XWiki < 7.1 use #set ($statuses = $mailResult.statusResult.getByState('FAILED'))
#set ($statuses = $mailResult.statusResult.getAllErrors())
#if ($statuses.hasNext())
  #set ($status = $statuses.next())
  {{error}}
    Error: $status.errorSummary ($status.state)

    $status.errorDescription
  {{/error}}
#end
{{/velocity}}

4. Add a valid email adress in the code and save the page.
5. Check your email inbox.
6. Check the Mail sending statuses in administration.
7.Check the console.

Actual result:

• After you access the page containing that code , you get the email in your inbox.
• The problem is in the console, that generates multiple errors:

  2016-08-01 12:05:27,148 [Mail Sende2r Thread] ERROR o.x.m.i.DatabaseMailListener   - Error when looking for a previous mail status for message [wf6IcqpIFAxhHxKKHDZ37sZ5zoE=] of batch [7c9b4454-91a2-441f-a0a1-5e73f2e74afd].
  org.xwiki.mail.MailStoreException: Failed to load mail statuses matching the filter [{id=wf6IcqpIFAxhHxKKHDZ37sZ5zoE=}] from the database.
          at org.xwiki.mail.internal.DatabaseMailStatusStore.load(DatabaseMailStatusStore.java:142)
          at org.xwiki.mail.internal.DatabaseMailStatusStore.load(DatabaseMailStatusStore.java:98)
          at org.xwiki.mail.internal.DatabaseMailListener.retrieveExistingMailStatus(DatabaseMailListener.java:193)
          at org.xwiki.mail.internal.DatabaseMailListener.onSendMessageSuccess(DatabaseMailListener.java:126)
          at org.xwiki.mail.internal.thread.SendMailRunnable.sendMail(SendMailRunnable.java:184)
          at org.xwiki.mail.internal.thread.SendMailRunnable.runInternal(SendMailRunnable.java:117)
          at org.xwiki.mail.internal.thread.SendMailRunnable.run(SendMailRunnable.java:78)
          at java.lang.Thread.run(Unknown Source)
  Caused by: com.xpn.xwiki.XWikiException: Error number 3301 in 3: Exception while switching to database null
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:758)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:932)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:831)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.execute(XWikiHibernateBaseStore.java:1323)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.executeRead(XWikiHibernateBaseStore.java:1391)
          at org.xwiki.mail.internal.DatabaseMailStatusStore.load(DatabaseMailStatusStore.java:122)
          ... 7 common frames omitted
  Caused by: java.lang.NullPointerException: null
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getXWikiConfig(AbstractDataMigrationManager.java:300)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDBVersionFromConfig(AbstractDataMigrationManager.java:375)
          at com.xpn.xwiki.store.migration.hibernate.HibernateDataMigrationManager.getDBVersionFromDatabase(HibernateDataMigrationManager.java:88)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDataMigrationStatus(AbstractDataMigrationManager.java:421)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDatabaseStatus(AbstractDataMigrationManager.java:564)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.checkDatabase(AbstractDataMigrationManager.java:532)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:754)
          ... 12 common frames omitted
  2016-08-01 12:05:27,209 [Mail Sender Thread] WARN  o.x.m.i.DatabaseMailListener   - Forcing a new mail status for message [wf6IcqpIFAxhHxKKHDZ37sZ5zoE=] of batch [7c9b4454-91a2-441f-a0a1-5e73f2e74afd] to send_success state.
  2016-08-01 12:05:27,246 [Mail Sender Thread] ERROR o.x.m.i.DatabaseMailListener   - Failed to delete mail status [messageId = [wf6IcqpIFAxhHxKKHDZ37sZ5zoE=], batchId = [7c9b4454-91a2-441f-a0a1-5e73f2e74afd], state = [send_success], date = [Mon Aug 01 12:05:27 EEST 2016], recipients = [irina.hrehorciuc@xwiki.com]] from the database
  org.xwiki.mail.MailStoreException: Failed to delete mail status (message id [wf6IcqpIFAxhHxKKHDZ37sZ5zoE=]) from the database.
          at org.xwiki.mail.internal.DatabaseMailStatusStore.delete(DatabaseMailStatusStore.java:207)
          at org.xwiki.mail.internal.DatabaseMailListener.deleteStatus(DatabaseMailListener.java:227)
          at org.xwiki.mail.internal.DatabaseMailListener.onSendMessageSuccess(DatabaseMailListener.java:141)
          at org.xwiki.mail.internal.thread.SendMailRunnable.sendMail(SendMailRunnable.java:184)
          at org.xwiki.mail.internal.thread.SendMailRunnable.runInternal(SendMailRunnable.java:117)
          at org.xwiki.mail.internal.thread.SendMailRunnable.run(SendMailRunnable.java:78)
          at java.lang.Thread.run(Unknown Source)
  Caused by: com.xpn.xwiki.XWikiException: Error number 3301 in 3: Exception while switching to database null
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:758)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:932)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:831)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.execute(XWikiHibernateBaseStore.java:1323)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.executeWrite(XWikiHibernateBaseStore.java:1437)
          at org.xwiki.mail.internal.DatabaseMailStatusStore.delete(DatabaseMailStatusStore.java:195)
          ... 6 common frames omitted
  Caused by: java.lang.NullPointerException: null
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getXWikiConfig(AbstractDataMigrationManager.java:300)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDBVersionFromConfig(AbstractDataMigrationManager.java:375)
          at com.xpn.xwiki.store.migration.hibernate.HibernateDataMigrationManager.getDBVersionFromDatabase(HibernateDataMigrationManager.java:88)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDataMigrationStatus(AbstractDataMigrationManager.java:421)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDatabaseStatus(AbstractDataMigrationManager.java:564)
          at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.checkDatabase(AbstractDataMigrationManager.java:532)
          at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:754)
          ... 11 common frames omitted
  

• If you access multiple times the test page, you will get all the emails, but in the Mail sending status all the statuses are prepare_success instead of sent_success (check the expected behavior in this tutorial https://www.youtube.com/watch?v=1ffnSZeR-So)
• Also, if you add a wrong port to the email configuration, save it and access the test page, you will get in Mail sending status a new entry also with status prepare_success instead of send_error