Getting this exception in Symfony 2.5.5 with Swiftmailer 5.3.0. I'm following the cookbook example exactly. The error is thrown when calling MessageDataCollector#getMessages():
// Check that an e-mail was sent
$this->assertEquals(1, $mailCollector->getMessageCount());
$collectedMessages = $mailCollector->getMessages();
$message = $collectedMessages[0];
The message count assertion is also failing with a value of zero.
As far as I can tell the collector is failing to do any actual collecting in the action. Any ideas?
I encountered the same problem after I applied kriswallsmith's test optimization trick. I could see the result of the mail sending in the web profiler when running the development version, but couldn't get the data in the testing environment.
After applying Kris' trick I noticed that the swiftmailer.mailer.default.plugin.messagelogger service was not registered with the container during the test, so the collect() method of the MessageDataCollector class did not log the data for the mail sending. This is why it wasn't possible to retrieve information from the swiftmailer collector.
The solution is either not to overrride the initializeContainer() method in AppKernel.php or to override it, but making sure messagelogger service is available for test cases that send mails.