Unclear validation error when user forgets to upload csv dataset file

When you're creating CSV dataset and you forget to upload the file after selecting it and you click Test button, there's validation error "may not be null", which is not very informative. Could you please improve it like "You must upload the file using the button on the right".

Even better would be if the file was automatically uploaded after user selects the file (I already heard several people complaining about forgetting to click upload button after selecting the file).