1. What was the http status you get when you print xmlhttp.status ?
2. Does it also print null when you print xmlhttp ?
3. Have you tried reading remote or local json object instead ?
A few thoughts.
First, I would not recommend putting anything under the application folder as it appears you are doing here. Please use the UserDrive folder instead. The \Application folder is meant to be used by system software and will be wiped out every time the OS needs to be updated. Also, if you filled it or corrupted it somehow, the device could end up being hard or impossible to recover.
Depending on how you have your apps.json set, you might possibly have a cross domain issue. You would want it set like
One thing that might help is to pull the log.txt file from the root of the userdrive and take a look at that. That will many times show an error that might help you figure out what the issue is.