CheckedComboBox Custom Control WPF

CheckedComboBox.cs

using System;
using System.Collections;
using System.Text;
using System.Windows;
using System.Windows.Controls;

namespace CryptoTools.UI.Controls
{
    
    public class CheckedComboBox : ComboBox
    {
        private ListBox _listbox;

        static CheckedComboBox()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(CheckedComboBox), new FrameworkPropertyMetadata(typeof(CheckedComboBox)));
        }

        public static readonly DependencyProperty DefaultTextProperty =
            DependencyProperty.Register("DefaultText", typeof (String), typeof (CheckedComboBox), new PropertyMetadata(default(String)));

        public String DefaultText
        {
            get { return (String) GetValue(DefaultTextProperty); }
            set { SetValue(DefaultTextProperty, value); }
        }

        public static readonly DependencyProperty SelectedItemsTextProperty =
            DependencyProperty.Register("SelectedItemsText", typeof(String), typeof(CheckedComboBox), new FrameworkPropertyMetadata()
                                                                                                          {
                                                                                                              BindsTwoWayByDefault = true
                                                                                                          });

        public String SelectedItemsText
        {
            get { return (String)GetValue(SelectedItemsTextProperty); }
            set { SetValue(SelectedItemsTextProperty, value); }
        }

        public static readonly DependencyProperty DelimeterProperty =
            DependencyProperty.Register("Delimeter", typeof (String), typeof (CheckedComboBox), new PropertyMetadata(","));

        public String Delimeter
        {
            get { return (String) GetValue(DelimeterProperty); }
            set { SetValue(DelimeterProperty, value); }
        }

        public IList SelectedItems
        {
            get { return _listbox.SelectedItems; }
        }

        protected override void OnDropDownClosed(EventArgs e)
        {
            base.OnDropDownClosed(e);
            ToolTip = GetFormattedText(Environment.NewLine);
        }
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            
            if (this.Template != null)
            {
                _listbox = (ListBox)Template.FindName("ListBox", this);
                _listbox.SelectionChanged += (sender, args) =>
                                                 {
                                                     OnSelectionChanged(args);
                                                     UpdateText();
                                                     //Console.WriteLine("========");
                                                     //foreach (var selectedItem in _listbox.SelectedItems)
                                                     //{
                                                     //    Console.WriteLine(selectedItem);
                                                     //}
                                                     //Console.WriteLine("========");
                                                 };
            }
        }

        private void UpdateText()
        {
            if(_listbox == null || _listbox.SelectedItems.Count == 0)
            {
                SelectedItemsText = DefaultText;
                return;
            }
            Console.WriteLine(GetFormattedText(Delimeter));
            SelectedItemsText = GetFormattedText(Delimeter);
        }

        private String GetFormattedText(String delimeter)
        {
            var sb = new StringBuilder();
            foreach (var selectedItem in _listbox.SelectedItems)
            {
                sb.Append(selectedItem).Append(delimeter);
            }
            //chop off the last delimeter
            sb.Remove(sb.Length - delimeter.Length, delimeter.Length);
            return sb.ToString();
        }
    }
}

Generic.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:CryptoTools="clr-namespace:CryptoTools"
    xmlns:Controls="clr-namespace:CryptoTools.UI.Controls">

     <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="../Resources/Colors.xaml" />
        </ResourceDictionary.MergedDictionaries>

        <Style TargetType="{x:Type Controls:CheckedComboBox}">
        <Setter Property="SnapsToDevicePixels" Value="true" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
        <Setter Property="ScrollViewer.CanContentScroll" Value="true" />
        <Setter Property="MinWidth" Value="120" />
        <Setter Property="MinHeight" Value="20" />
        <Setter Property="Foreground" Value="{StaticResource ForegroundBrush}" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="Controls:CheckedComboBox">
                    <Grid x:Name="MainGrid">
                        <ToggleButton Name="ToggleButton" Template="{StaticResource ComboBoxToggleButton}"
                                      Grid.Column="2" Focusable="false"
                                      IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}" ClickMode="Press">
                        </ToggleButton>
                        <ContentPresenter Name="ContentSite" IsHitTestVisible="False"
                                          Content="{TemplateBinding SelectedItemsText}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                          ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Margin="3,3,23,3" VerticalAlignment="Center"
                                          HorizontalAlignment="Left" />
                        <TextBox x:Name="PART_EditableTextBox" CaretBrush="White" Foreground="White" Style="{x:Null}"
                                 Template="{StaticResource ComboBoxTextBox}" HorizontalAlignment="Left" VerticalAlignment="Center" Margin="3,3,23,3"
                                 Focusable="True" Background="Transparent" Visibility="Hidden" IsReadOnly="{TemplateBinding IsReadOnly}" />
                        <Popup Name="Popup" Placement="Bottom" IsOpen="{TemplateBinding IsDropDownOpen}"
                               AllowsTransparency="True" Focusable="False" PopupAnimation="Slide">
                            <Grid Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}"
                                  MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border x:Name="DropDownBorder" Background="{StaticResource BackgroundBrush}"
                                        BorderThickness="1" BorderBrush="{StaticResource BorderBrush}" />
                                <ScrollViewer Name="ScrollView" Margin="4,6,4,6" SnapsToDevicePixels="True">
                                    <ListBox Name="ListBox" ItemsSource="{Binding ItemsSource,RelativeSource={RelativeSource AncestorType=Controls:CheckedComboBox}}" SelectionMode="Multiple">
                                        <ListBox.ItemTemplate>
                                            <DataTemplate>
                                                <CheckBox Content="{Binding Content,RelativeSource={RelativeSource AncestorType=ListBoxItem}}" IsChecked="{Binding IsSelected,RelativeSource={RelativeSource AncestorType=ListBoxItem}}" Name="PART_Checkbox" />
                                            </DataTemplate>
                                        </ListBox.ItemTemplate>
                                    </ListBox>
                                </ScrollViewer>
                            </Grid>
                        </Popup>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="HasItems" Value="false">
                            <Setter TargetName="DropDownBorder" Property="MinHeight" Value="95" />
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="false">
                            <Setter Property="Foreground" Value="{StaticResource DisabledBrush}" />
                        </Trigger>
                        <Trigger Property="IsGrouping" Value="true">
                            <Setter Property="ScrollViewer.CanContentScroll" Value="false" />
                        </Trigger>
                        <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
                        </Trigger>
                        <Trigger Property="IsEditable" Value="true">
                            <Setter Property="IsTabStop" Value="false" />
                            <Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible" />
                            <Setter TargetName="ContentSite" Property="Visibility" Value="Hidden" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers></Style.Triggers>
    </Style>
</ResourceDictionary>

ComboBoxToggleButton not exit
& it gives me error
'System.Windows.Style' is not a valid value for property 'Template'.

i don't know how to use it in right way
thanks in advance