"File already imported" causes crash

[mlisle]

We've had a couple of users report this issue with the iOS app (and I've been able to reproduce). If you tap the import-certificate link after having already imported the blockcert into your wallet, the following happens:

1. the blockcerts app opens
2. the "File already imported" prompt appears momentarily
3. the app crashes

I'm on iOS 12.1.1 (also tried on 12.1), using app version 3.1.