Application main has not been registered

The problem is you haven't opened your project in Android Studio. The previous project is running in android studio and you are running the latest one in emulator.

I created a Project from "Doco", and save it with Practice. so write that line like that

AppRegistry.registerComponent('Practice', () => FixedDimensionsBasics);

then I get an error like in a question

So, I check on my Appdelegate.m file on ios folder and find that Project name is Project so I change that line with

AppRegistry.registerComponent('Project', () => FixedDimensionsBasics);

and my Error solved

so if you are using "Doco" then check that thing.

The following solution worked:

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

Credits: Rishav Kumar

(Cannot you people just reply to mark as solve ?)