AngularJS error: 'argument 'FirstCtrl' is not a function, got undefined'

I got exactly the same error message and in my case it turned out i didn't list the controller JS file (e.g. first-ctrl.js) in my index.html

I just did this tutorial and followed @gion_13 answer. Still did not work. Solved it by making my ng-app name in the index identical to the one in my js file. Exactly identical, even the quotes. So:

<div ng-app="myapp">
    <div ng-controller="FirstCtrl">

and the js:

 angular.module("myapp", [])
.controller('FirstCtrl',function($scope) {
    $scope.data= {message:"hello"};
  });

Weird how the ng-app has to be identical but the ng-controller doesn't.

You have 2 unnamed ng-app directives in your html.
Lose the one in your div.

Update
Let's try a different approach.
Define a module in your js file and assign the ng-appdirective to it. After that, define the controller like an ng component, not as a simple function:

<div ng-app="myAppName">  
<!-- or what's the root node of your angular app -->

and the js part:

angular.module('myAppName', [])
    .controller('FirstCtrl', function($scope) {
         $scope.data = {message: 'Hello'};
    });

Here's an online demo that is doing just that : http://jsfiddle.net/FssbL/1/