firebase-ios-sdk: crash in +[FRepoManager createRepo:config:database:] stemming from initializing Database.database()

Environment

• Xcode version: 8.3.2
• Firebase SDK version: 4.0.0
• Library version: unsure what Library is
• Firebase Product: database

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

Crash in +[FRepoManager createRepo:config:database:]. Here’s the relevant part of the stack trace:

0   CoreFoundation                      0x000000010b386b0b __exceptionPreprocess + 171
1   libobjc.A.dylib                     0x0000000109f55141 objc_exception_throw + 48
2   CoreFoundation                      0x000000010b2a16e2 -[__NSDictionaryM setObject:forKey:] + 1042
3   wrytr                               0x00000001088d7f83 +[FRepoManager createRepo:config:database:] + 404
4   wrytr                               0x00000001088bed22 -[FIRDatabase ensureRepo] + 151
5   wrytr                               0x00000001088be3eb -[FIRDatabase reference] + 31
6   wrytr                               0x0000000108854471 _TFCC5wrytr8Firebase8ProvidercfT_S1_ + 97

It seems to be related to https://github.com/firebase/quickstart-ios/issues/261, but the fix provided there (be on the main thread) doesn’t apply to me, as my code is executing on Thread 1:

I know the crash is happening somewhere inside a dictionary set in this method in FRepoManager, but I’m not sure why

Relevant Code:

https://github.com/AndrewSB/wrytr/blob/master/Common/API/FirebaseProvider.swift#L13