I am trying out the new Firebase/Crashlytics setup and seem to run into a weird issue.
I had the same issue and it took me a lot of time to figure out.
I solved my problem by going to FireBase Console > [my project] > Crashlytics. In the dropdown you need to select your app. After that a wizard with three steps will show up.
Click whatever option suits you, I chose the first because it's a new app. After that the second step shows up.
Now here comes the important part. Make sure you click the link to go to the documentation! A new browser tab will open up. If you already finished the project setup like me, you can close it. After that you will automatically go to the 3rd option. Most of the time I never clicked the link because I had already completed the installation. And once I did I never noticed that the 3rd option was available because I'm at the other tab. As a result I never got to see the 3rd option.
At this point you should open your app and it will automatically connect to Firebase without any problem.
Edit: After I tried to add some other build variants I noticed that I was still getting some errors while retrieving the settings. It took me a lot of time to figure out how to solve it. The best thing I could come up with is:
It also helped me to enable additional debug logging for fabric and crashlytics.
adb shell setprop log.tag.Fabric DEBUG
adb shell setprop log.tag.CrashlyticsCore DEBUG