I ran into this error when doing this tutorial. It was because I had copied the wrong files into the project at the second step (Module Two, Initialize Amplify at Runtime). Verify that in your project has the correct configuration files included "amplifyconfiguration.json" and "awsconfiguration.json". I accidentally included the amplifytools.xcconfig and ran into the webDomain field being blank, causing the project to crash. Hope this helps someone.