Ran into the same issue, when I opened Xcode I received a notifications about needing to install additional components. After installing the updates, I was able to run the Ionic project again.
Update
Actually, I was mistaken, looks like it was @Sujan12 suggest on "Error: Cannot read property ‘replace’ of undefined" for ios-sim that actually fixed the issue for me.
in your project folder root, do cd platforms/ios/cordova && npm install ios-sim