what should be a file download path

Thanks for sharing that error output! That shows that you're getting a 'NullPointerException' for the 'File result' in your 'onPostExecute'. That is, the object itself is null, which is not the same as saying the downloaded file has zero length.

You'll need to investigate why your code isn't actually returning the 'file' object. I suspect it's because something is going wrong with the downloading or writing of the file, but, upon closer inspection of your code, it seems you're not seeing it because you're silently throwing away potential exceptions here:

} catch (DbxException | IOException e) {
}

You should add error handling there to take handle any issues that may arise (e.g., if you don't have permission to write to the local file path you specified, etc.).