java.lang.IllegalArgumentException: Crashlytics found an invalid API key: null.

I followed the SETUP from CONTRIBUTING.md to fill in dummy values, checked out the dev branch, and selected the freeDebug flavor, but am still running into the following error when building:

java.lang.IllegalArgumentException: Crashlytics found an invalid API key: null.

[AaronRietschlin]

So, I realized that the process was failing when generating Resources for the Amazon product flavor. As such, I added the following line of code to the amazon product flavor bracket in the SeriesGuide/build.gradle:

ext.enableCrashlytics = false

While that worked for that build flavor, it failed again at the same spot during the free product flavor. As such, I added it to the debug buildType so that my debug buildType looks like the following:

debug {
    pseudoLocalesEnabled true
    ext.enableCrashlytics = false
}

While this may not be what we want to commit, it will allow you to build the application.

[tilalapradip]

This post is helpful https://stackoverflow.com/questions/27472452/crashlytics-found-an-invalid-api-key-androidstudio-build/50754641#50754641