Preparing for release of 2.15.13
Added null exception check
Merge branch 'hotfix/2.15.13' of https://github.com/shouldit/android-proxy into hotfix/2.15.13

# By Marco Pagliari
# Via Marco Pagliari
* 'hotfix/2.15.13' of https://github.com/shouldit/android-proxy:
  Added comment to circle ci config