pop to the main view controller when topic is changed
avoid the crash when no mail account is set

  * Problem with MFMailComposeViewController's “No mail accounts” alert - SDK 3.0 vs SDK 4.0
    http://stackoverflow.com/questions/3086038/problem-with-mfmailcomposeviewcontrollers-no-mail-accounts-alert-sdk-3-0-vs#answer-3088419