Cordova sqliteplugin karma test description

Hello I want to implement some test cases for my ionic framework application which uses cordova sqliteplugin to get data from a sqlite database.

I'm very new in writing test cases for angularjs.

Well in my app.js I open the database with this:

angular.module('starter', ['ionic','starter.services', 'starter.controllers', 'starter.config','ngCordova'])

.run(function($ionicPlatform, $cordovaSQLite, $ionicLoading) {
  $ionicPlatform.ready(function() {

    var dbName = 'testDB.db';
    if(window.cordova) {
      window.plugins.sqlDB.copy(dbName);
      db = $cordovaSQLite.openDB(dbName);
    }

    ....

My services.js are looking like this:

angular.module('starter.services', ['ngCordova'])

.factory('ProductService',function($cordovaSQLite){

    var getAllProducts = function() {
        var productList = [];

        var query = "SELECT c.name as name, c.id as id FROM cars as c ORDER BY c.name";
        $cordovaSQLite.execute(db, query, []).then(function(res) {

            if(res.rows.length > 0) {

                for(var i = 0; i < res.rows.length; i++) {
                    productList.push({id: res.rows.item(i).id, name: res.rows.item(i).name});
                }

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

        return productList;
    };

    return {
        getAllProducts: getAllProducts
    };

});

And my test description to test the service looks like this:

describe('ProductService unit tests', function () {
    var ProductService;

    beforeEach(module('starter.services'));
    beforeEach(angular.mock.module('starter'));

    beforeEach(inject(function(_ProductService_) {
        ProductService = _ProductService_;
    }));

    it('can get an instance', inject(function(ProductService) {
        expect(ProductService).toBeDefined();
    }));

    //THIS TEST FAILS
    it('should get some products from the database', inject(function(ProductService) {
        expect(ProductService.getAllProducts().length).toBeGreaterThan(0);
    }));
});

If I run the test I get:

TypeError: 'null' is not an object (evaluating 'n.transaction')

I think its because the database is not really initialized during the test, so I tried to mock the starter app but it didn't solve the problem. I tried to google on this but could not find any examples or tutorials for karma with sqlite and angularjs.

Thanks for any help.