Yesterday, I received numerous reports that the robots.txt tester tool in the Google Search Console was broken. Google indeed confirmed it and said they are working on a fix. Well, Google indeed fixed it.
Here is what happened when you tried loading the tool, it just loaded a blank box:
I did test it this morning, and it all seems back to working order.
The first report came in around 8:30am ET yesterday, I guess it stopped working around then. The issues and complaints continued throughout most of the day yesterday.