Can't start BitNinja because an instance is already running

Solution

1. Just echo an empty string to the file.
   echo "" > /var/lib/bitninja/blue.pid.
2. Try to start the BitNinja agent with service bitninja start command

If that does not work for some reason:

1. Delete the whole file with the following command
   rm  /var/lib/bitninja/blue.pid -y 
2. And then restart BitNinja with the service bitninja restart command