run time exception: 'StaticResource mySelector' is not a valid value for property 'ItemTemplateSelector'

Question

trying to use the ItemTemplateSelector property of ListBox to control which DataTemplate is bound to a ListBox Item.

When I run the code getting an exception:
'StaticResource mySelector' is not a valid value for property 'ItemTemplateSelector'

How to use code to control which data template to apply to the binding to items in a ListBox?

Here is the code:

    <ListBox x:Name="lstItems" Grid.Row="1" Grid.Column="0"
             ItemTemplateSelector="StaticResource mySelector"
             ItemsSource="{Binding SystemList}">
   </ListBox>

<Window x:Class="WpfWrkmbrpdm.Windows.SelectSystemWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:local="clr-namespace:WpfWrkmbrpdm.Windows"
        Title="SelectSystemWindow" Height="400" Width="400">

associate "mySelector" with the type TaskListDataTemplateSelector.

Also, define a DataTemplate with a Name and DataType. I want this template to be the default template.

    <Window.Resources>
        <local:TaskListDataTemplateSelector x:Key="mySelector" />
        
        <DataTemplate x:Name="defaultTemplate" DataType="{x:Type as:As400System}">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="auto"></RowDefinition>
                </Grid.RowDefinitions>

                <StackPanel Orientation="Horizontal"
                        Grid.Row="0" Grid.Column="1">
                    <Label>System name</Label>
                    <TextBox Text="{Binding SystemName}"
                       MinWidth="60" Margin="3"/>
                </StackPanel>
            </Grid>
        </DataTemplate>
    </Window.Resources>

The code for the DataTemplateSelector class:

  public class TaskListDataTemplateSelector : DataTemplateSelector
  {
    public override DataTemplate
        SelectTemplate(object item, DependencyObject container)
    {
      FrameworkElement element = container as FrameworkElement;

      if (element != null && item != null && item is As400System)
      {
        var taskitem = item as As400System;

        if (taskitem.SystemName == "Add")
          return
              element.FindResource("importantTaskTemplate") as DataTemplate;
        else
          return
              element.FindResource("defaultTemplate") as DataTemplate;
      }

      return null;
    }
  }