Have a look at the error logs in the Studio Pro console if you are running this locally. It should say what is going wrong.
I would also suggest checking the return value of Base64 Decode to File. It should return true if it was able to decode the image. If it's false, you could handle this error and return a suitable error message to the user.
Good luck!
This is what I see as an error that the source file is null for the JavaAction.
This is how the JavaAction has been set.