Fix app_by_short_name to check for None obj.
Fix app_by_short_name to check for None obj.

harihpr/tweetclickers,OpenNewsLabs/pybossa,inteligencia-coletiva-lsd/pybossa,harihpr/tweetclickers,Scifabric/pybossa,jean/pybossa,PyBossa/pybossa,PyBossa/pybossa,geotagx/pybossa,jean/pybossa,OpenNewsLabs/pybossa,geotagx/pybossa,stefanhahmann/pybossa,Scifabric/pybossa,inteligencia-coletiva-lsd/pybossa,stefanhahmann/pybossa