Error: Angular is undefined

Question

Hi,
I am trying to use Angular JS to bind a dropdown from SharePoint custom list.
I have tried the below code:
// visual webpart

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/angularjs/1.3.9/angular.min.js"></script>
 
<script src="/_layouts/Test/FillData.js" type="text/javascript"></script>
 
<SharePoint:FormDigest ID="FormDigest1" runat="server">
</SharePoint:FormDigest>

<div ng-app="mainApp" ng-controller="mainController">
<form novalidate >

 <select ng-model="ddlTowers" ng-options="Tower.Name for Tower in Towers track by Tower.Id" required>
         <option value="">Select</option>
        </select>
 
//FillData.js

var app = angular.module('mainApp', []);
     app.controller('mainController', function ($scope) {

         ExecuteOrDelayUntilScriptLoaded(GetData, "sp.js");

         $scope.Towers = null;
         $scope.Towers.items = [];

         function GetData() {

             var clientContext = new SP.ClientContext.get_current();
             var web = clientContext.get_web();
             clientContext.load(web);
             var list = web.get_lists().getByTitle('Towers');
             var query = SP.CamlQuery.createAllItemsQuery();
             var listitems = list.getItems(query);
             clientContext.load(listitems, "Include(Title)");
             clientContext.executeQueryAsync(OnSucceeded, onFailed);

             function OnSucceeded() {
                 $scope.$apply(function () {
                     var itemsCount = listitems.get_count();
                     for (i = 0; i < itemsCount; i++) {
                         var listitem = listitems.itemAt(i);
                         $scope.Towers.items.push({ "title": listitem.get_fieldValues().Title });
                     }
                 });
             }

             function onFailed(sender, args) {
                 alert('Error');
             }
         }
     });

I am getting error as "Angular is undefined." The control is going to filldata.js file and is breaking in the first line itself.

How to fix this?

Thanks