FirebaseApp with name [DEFAULT] doesn't exist

Please do double check, you added

apply plugin: 'com.google.gms.google-services' 

at the bottom of app's gradle file and then clean and rebuild the project

Not sure, if it is relevant here. But there is another scenario when this crash can happen.

---

If your app has a service (with different process) and you're creating your own Application class, the service and the foreground app will use the same Application class (not same instance) to initialize. Now when I am using com.google.firebase:firebase-crash dependancy to handle crashes, it creates a background service your.app.packagename:background_crash. For some reason, this was inducing crashes on my app. Specifically, because in my Application class, I was making a call like,

FirebaseDatabase.getInstance().setPersistenceEnabled(true);

I am assuming, the background service when initing with our Application class, somehow Firebase is not initialized. To fix this, I did

if (!FirebaseApp.getApps(this).isEmpty())
        FirebaseDatabase.getInstance().setPersistenceEnabled(true);

I've had similar problem, and for me it was a bug/problem with manifest merger. I've found out that FirebaseInitProvider has not been injected into final manifest file because of tools:node="replace" in my app's manifest file. So, try to remove this xml tag and FirebaseInitProvider will be injected and Firebase can be initialized properly.