Jasmine angular unit test 'Cannot read 'property' of undefined

I have just started learning angular unit testing. However, this test on a function with http call fails. I have pin pointed the problem but however I am not being able to fix it. I know it's some simple issue

Controller

//Get data from URL
vm.getJson = function() {
    var url = 'http://ift.tt/1KEkUhj',
        count = 0;
    $http.get(url).success(function(response) {
        console.log(response);
        for (var i = 0; i < response.data.children.length; i++) {
            vm.data.push(response.data.children[i].data);
            count++;
            if (count === response.data.children.length) {
                vm.numberOfPages();
            }
        }

        vm.result = true;

    }).error(function(err) {
        console.log(err);
    });

};

The response I am getting is:

Spec

 //Testing the getJson function
describe('vm.getJson()', function() {

   it('It should return dummy Data as response and vm.result to be truthy', function() {

    var dummyData = {name: 'Umair'};
    $httpBackend.whenRoute('GET','http://ift.tt/1KEkUhj').respond(200, dummyData);

    MainCtrl.getJson(); 

    $httpBackend.flush();

    expect(MainCtrl.result).toBeTruthy();


}); });

I don't get any errors and the test passes if I remove the loop from the controller function. The error I am getting is:

Cannot read 'Children' of undefined. From the image I have attached with the response data, children is the array.