Do we need app key to get file of other user

I am getting this error in android sdk Getting ‘Uncaught (in promise) null’ error
when calling getStringFileFromUser

You only need the app key if the file you’re requesting is encrypted and/or signed, and you have requested to decrypt and/or verify it. If you’re just getting the file data, then no app key should be required.

I am getting this error. Can you tell me what went wrong