The job has no provider assigned

The following code block in MicrosoftTranslator::doRequest results in a TMGMTException('The job has no provider assigned.');

          $result = $this->doRequest($job->getTranslator(), 'Translate', array(
            'from' => $job->getRemoteSourceLanguage(),
            'to' => $job->getRemoteTargetLanguage(),
            'contentType' => 'text/plain',
            'text' => $value['#text'],
          ), array(
            'Content-Type' => 'text/plain',
          ));

The job doesn't appear to have a translator set. This issue only seems to occur for tmgmt_locale strings.

Steps to reproduce:
- make sure you have 2 providers, file and microsoft (in that order)
- go to sources
- select locale
- check a label
- request translation
- select the microsoft plugin
- submit
=> this will result in the exception.

When refreshing (re-submitting) the form, all translations appear.

Something is going wrong with the ajax call when changing the provider (from file to microsoft).