Failure to call AppRegistry.registerComponent

To elaborate on what oblador said, I was still having this issue after closing all the terminal windows/killing all the other React Native processes. What happened to me was the value of the moduleName in AppDelegate.m (in the root of the XCode project) was incorrect. I am using a build from Barton Hammond's Snowflake and was trying to rename it from "snowflake" to my own and I missed this value and was getting that error.

It is likely that you already have another React Native packager server running for another app or that the app name in AppDelegate.m and your index.js is not the same.

Try closing all terminal windows, double check the value of moduleName and run the project again.

In my case, it is caused by the case sensitivity of the app name in index.ios.js.

It works after I changed from

AppRegistry.registerComponent('AppleReactNative', () => App)

to

AppRegistry.registerComponent('applereactnative', () => App)

For us, the problem was that the name of the app registered did not match the root folder. So, if your root folder is /ChatApp, then register your app as so:

AppRegistry.registerComponent('ChatApp', ()  => App);