Few hours ago I had the same error during taking my first steps with Firebase. I had no idea how to find solution but after few hours of debugging I noticed that the problem occurs when I had initialized project with Firebase using dependency manager built-in in the Xcode and then I tried to initialize Firestore using CocoaPods. To be more specific, first thing I do was Firebase installation using this one:
Then, I started using CocoaPods like:
$ pod init
$ vim Podfile // adding 'firebase/firestore' dependency or specific line from https://github.com/firebase/firebase-ios-sdk
$ pod install
$ open <ProjectName>.xcworkspace
In the next step, I tried to check basic Firestore operations like creating collection and documents with data and when I run the app - I got similar error. It was an exception during app run.
I think in my case the problematic thing was possible duplicates because of supplying the project with a Firebase (with all dependencies) twice.
So, I removed dependency installed from the Xcode manager. In my Info.plist there is no dependencies:
Then, from the terminal I removed Pods directory and just called an update.
$ rm -rf Pods
$ pod update
All dependencies were re-installed and workspace has been recreated.
Now, after these steps everything works fine.
