RefererNotAllowedMapError in console, and the whole screen is gray. But, at the bottom of the Mashup specification, there is changelog where it says that we don't need to get an API key.
Should I obtain an API key?
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As mentioned in the comment, Mashup has been retired for the 2019 course. You of course can still work on it if you'd like, but if you are going for a certificate, it cannot be submitted for a grade.
If you do, you will need to get your own API key (note that Google now requires a credit card in order to receive a key).
If you are going for a certificate, be sure to visit the course's homepage and follow the instructions in the yellow box that explain how to transition from the 2018 to the 2019 course.