null reference PeoplePicker in a Sharepoint hosted app

Question

I am trying to integrate a peoplepicker in a SharePoint Hosted App as explained

here:

Code example: Using the client-side People Picker in an app for SharePoint

Unfortunately, when

 this.SPClientPeoplePicker_InitStandaloneControlWrapper(peoplePickerElementId, null, schema);

is called in function initializePeoplePicker

this is null

Could you tell me why? Thanks

App.js

    'use strict';


var context = SP.ClientContext.get_current();
var user = context.get_web().get_currentUser();

// This code runs when the DOM is ready and creates a context object which is needed to use the SharePoint object model
$(document).ready(function () {
    getUserName();
    initializePeoplePicker('peoplePickerDiv');

    function initializePeoplePicker(peoplePickerElementId) {

        // Create a schema to store picker properties, and set the properties.
        var schema = {};
        schema['PrincipalAccountType'] = 'User,DL,SecGroup,SPGroup';
        schema['SearchPrincipalSource'] = 15;
        schema['ResolvePrincipalSource'] = 15;
        schema['AllowMultipleValues'] = true;
        schema['MaximumEntitySuggestions'] = 50;
        schema['Width'] = '280px';

        // Render and initialize the picker. 
        // Pass the ID of the DOM element that contains the picker, an array of initial
        // PickerEntity objects to set the picker value, and a schema that defines
        // picker properties.
        this.SPClientPeoplePicker_InitStandaloneControlWrapper(peoplePickerElementId, null, schema);
    }
});

// This function prepares, loads, and then executes a SharePoint query to get the current users information
function getUserName() {
    context.load(user);
    context.executeQueryAsync(onGetUserNameSuccess, onGetUserNameFail);
}

// This function is executed if the above call is successful
// It replaces the contents of the 'message' element with the user name
function onGetUserNameSuccess() {
    $('#message').text('Hello ' + user.get_title());
}

// This function is executed if the above call fails
function onGetUserNameFail(sender, args) {
    alert('Failed to get user name. Error:' + args.get_message());
}





function getUserInfo() {

    // Get the people picker object from the page.
    var peoplePicker = this.SPClientPeoplePicker.SPClientPeoplePickerDict.peoplePickerDiv_TopSpan;

    // Get information about all users.
    var users = peoplePicker.GetAllUserInfo();
    var userInfo = '';
    for (var i = 0; i < users.length; i++) {
        var user = users[i];
        for (var userProperty in user) {
            userInfo += userProperty + ':  ' + user[userProperty] + '<br>';
        }
    }
    $('#resolvedUsers').html(userInfo);

    // Get user keys.
    var keys = peoplePicker.GetAllUserKeys();
    $('#userKeys').html(keys);
}

Answer 1

It works if I change

 this.SPClientPeoplePicker_InitStandaloneControlWrapper(peoplePickerElementId, null, schema);

to

 SPClientPeoplePicker_InitStandaloneControlWrapper(peoplePickerElementId, null, schema);

though, the code comes from a Microsoft page