What is a "null" password?

I downloaded Tweeter on my IPhone 6, after setting up the account I was sent an email for verification at an outlook email account I have on my phone. I was then prompted to enter my outlook password in order to retrieve the verification email from Tweeter. I entered the password and a few seconds later I was asked to enter the "null" password. I had no idea what the phone prompt was asking. I do not know what a "null" password is nor why my iPhone needed it for installing a tweeter account. I took a picture with my iPad. I had to hit cancel and the prompted went away.

Below is the picture I took with my iPad:!t