Cannot invoke method containsKey() on null object

Until the bug is fixed by google, downgrading to the play services 4.0.1 seems to have fixed this issue for me so far:

classpath 'com.google.gms:google-services:4.0.1'

https://stackoverflow.com/a/50930048/4096987

From @tapchicoma comment to the original question, the issue: NullPointerException in Google Services Plugin now is marked as fixed for 4.2.0.

So, the final solution is to update to:

classpath 'com.google.gms:google-services:4.2.0'