Error Implementing Design Time Data Template

I want to display dummy data in the design window of a WPF GridView and found a similar example: Answer: Design time data for datatemplate in xaml - Stack Overflow

I created a new WPF App (.NET Framework 4.6.1) named WpfDataTemplate and copied in the code snippets from the example into MainWindow.xaml.cs and MainWindow.xaml. All additions are noted in comments with ##.

Unexpected Behavior:

1. The line <local:MyMockClass x:Key="DesignViewModel" /> produces the error "Object reference not set to an instance of an object."

2. The test text # strings are not displayed in the Designer.

3. The test text # strings are not displayed in the Window at runtime.

Questions:

1. What is the cause of the error?

2. What do I need to change for the code to work as expected?

---

MainWindow.xaml

<Window x:Class="WpfDataTemplate.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfDataTemplate"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
<!-- ## BEGIN ADD -->
<Window.Resources>
    <local:MyMockClass x:Key="DesignViewModel" />
</Window.Resources>
<!-- ## END ADD -->
<Grid>
    <!-- ## BEGIN ADD -->
    <ListBox x:Name="standardLayoutListBox" 
             d:DataContext="{Binding Source={StaticResource DesignViewModel}}"
             ItemsSource="{Binding MyListBoxItems}">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition />
                        <RowDefinition />
                    </Grid.RowDefinitions>
                    <Label Grid.Column="0" Content="{Binding text1}" />
                    <Label Grid.Column="1" Content="{Binding text2}" />
                </Grid>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
    <!-- ## END ADD -->
</Grid>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
// ## BEGIN ADD
using System.Collections.ObjectModel; 
// ## END ADD

namespace WpfDataTemplate
{
    // ## BEGIN ADD
    public class MyMockClass
    {
        public MyMockClass()
        {
            MyListBoxItems.Add(new MyDataClass() { text1 = "test text 1", text2 = "test text 2" });
            MyListBoxItems.Add(new MyDataClass() { text1 = "test text 3", text2 = "test text 4" });
        }
        public ObservableCollection<MyDataClass> MyListBoxItems { get; set; }
    }

    public class MyDataClass
    {
        public string text1 { get; set; }
        public string text2 { get; set; }
    }
    // ## END ADD

    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

Firstly you need to initialise your collection in your MyMockClass.

public class MyMockClass
{
    public MyMockClass()
    {
        MyListBoxItems = new ObservableCollection<MyDataClass>()
        MyListBoxItems.Add(new MyDataClass() { text1 = "test text 1", text2 = "test text 2" });
        MyListBoxItems.Add(new MyDataClass() { text1 = "test text 3", text2 = "test text 4" });
    }
    public ObservableCollection<MyDataClass> MyListBoxItems { get; set; }
}

I use the design instance in my Window like so:

d:DataContext="{d:DesignInstance Type=local:MyMockClass, IsDesignTimeCreatable=True}"