Null Reference Exception being thrown when using a data template selector in a user control WPF

Question

Hi im having an issue where a null Reference exception is being thrown in my xaml for a user control using a data template selector and I'm honestly not sure why. Here is the code below and any help would be appreciated.

UPDATE it only happens at design time not at run time

Data Template Selector code:

using System.Windows;
using System.Windows.Controls;
using UI.ViewModel;

namespace UI.DataTemplateSelectors
{
    public class CouponViewerDataTemplateSelector : DataTemplateSelector
    {
        public DataTemplate coupon1DataTemplateSelector { get; set; }
        public DataTemplate coupon2DataTemplateSelector { get; set; }
        public DataTemplate coupon3DataTemplateSelector { get; set; }



        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            if (item == null)
                return null;
            var CouponViewerViewModel = (CouponViewerViewModel)item;
            switch (CouponViewerViewModel.Coupon.CouponBarcode )
            {

                case "994460":
                    return Coupon1DataTemplateSelector;
                case "994470":
                    return Coupon2DataTemplateSelector;
                case "994480":
                    return Coupon3DataTemplateSelector;


            }
            return null;
        }


    }
}

The Xaml (I have collapsed the different DataTemplates code as the xaml file is massive)

<UserControl x:Class="UI.View.CouponViewerView"
        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:viewModel="clr-namespace:UI.ViewModel"
        xmlns:converter="clr-namespace:UI.View.Converter"
        xmlns:converters="clr-namespace:UI.Converters"
        xmlns:Web="clr-namespace:Web;assembly=Web"
        xmlns:dataTemplateSelectors="clr-namespace:UI.DataTemplateSelectors"
        mc:Ignorable="d"
        d:DataContext="{d:DesignInstance Type=viewModel:CouponViewerViewModel, IsDesignTimeCreatable=True}"


        Height="1018" Width="720">
    <UserControl.Resources>
        <converter:EmptyStringToVisibilityConverter x:Key="EmptyStringToVisibilityConverter"/>
        <converters:AppendCharToBeginningAndEndOfStringConverter x:Key="AppendCharToBeginningAndEndOfStringConverter"/>
        <converters:HexToBackgroundColourConverter x:Key="HexToBackgroundColourConverter"/>
        <converters:HexToForegroundColourConverter x:Key="HexToForegroundColourConverter"/>
        <converters:MaxCharcterSelectionNameConverter x:Key="MaxCharcterSelectionNameConverter"/>
        <converters:DateTimeToSplitDataGridConverter x:Key="DateTimeToSplitDataGridConverter"/>
        <converters:HeaderFormatterConverter x:Key="HeaderFormatterConverter"/>

            <DataTemplate x:Key="Coupon1DataTemplateSelector" ...>

            <DataTemplate x:Key="Coupon2DataTemplateSelector" ...>

            <DataTemplate x:Key="Coupon3DataTemplateSelector" ...>

                 <dataTemplateSelectors:CouponViewerDataTemplateSelector

        Coupon1DataTemplateSelector="{StaticResource Coupon1DataTemplateSelector}"
        Coupon2DataTemplateSelector="{StaticResource Coupon2DataTemplateSelector}"
        Coupon3DataTemplateSelector ="{StaticResource Coupon3DataTemplateSelector}"
        x:Key="CouponViewerDataTemplateSelector" />

</UserControl.Resources>

 <StackPanel>
        <StackPanel x:Name="StackPanelToPrint">
            <StackPanel.ContextMenu >
                <ContextMenu>
                    <MenuItem Header="Print"  Click="PrintCurrentForm" />
                    <MenuItem Header="OverLay Image"  Command="{Binding Path=OverlayImageToCouponViewerCommand }" />
                </ContextMenu>
            </StackPanel.ContextMenu>

            <Grid Margin="0,0,0,0">

                <TextBlock Text="{Binding DesignTimeError}" Visibility="{Binding DesignTimeError, Converter={StaticResource EmptyStringToVisibilityConverter}}" Grid.ColumnSpan="2" Margin="0,0,0,34" />
                <Canvas Grid.ColumnSpan="2" Margin="0,0,0,0">
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="11" Canvas.Top="128" />
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="742.5" Canvas.Top="128" />
                    <TextBlock Canvas.Left="338" FontSize="9" Foreground="Red" Canvas.Top="1046"  Text="{Binding Path=MarkSenseCoupon.CouponBarcode}" />
                    <TextBlock FontFamily="Free 3 Of 9"  Canvas.Left="24"  Canvas.Top="1026" FontSize="110" Height="15" Text="{Binding Path=Coupon.CouponBarcode,  Converter={StaticResource AppendCharToBeginningAndEndOfStringConverter}}"/>
                    <TextBlock Canvas.Left="409" FontSize="9" Foreground="Red"  Canvas.Top="1046" Text="{Binding Path=MarkSenseCoupon.CouponInstance}" />
                    <TextBlock FontFamily="Free 3 of 9" Canvas.Left="409" Canvas.Top="1026"  FontSize="110" Height="15" Text="{Binding Path=Coupon.CouponInstance,  Converter={StaticResource AppendCharToBeginningAndEndOfStringConverter}}"/>
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="11.5" Canvas.Top="1043" />
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="742.5" Canvas.Top="1043" />



                </Canvas>

                <Image Canvas.ZIndex="1" Source="{Binding Coupon.OverlayImage}"  Margin="0,-21,-76,108" Stretch="Fill" x:Name="PrintImageContextMenu" />




                <StackPanel Margin="-105,146,66,0" Height="900" VerticalAlignment="Top" >

                    <ContentControl Content="{Binding}"  ContentTemplateSelector="{StaticResource CouponViewerDataTemplateSelector}"  />


                </StackPanel>

            </Grid>
        </StackPanel>

    </StackPanel>

</UserControl>

Stack Trace:

    at UI.DataTemplateSelectors.CouponViewerDataTemplateSelector.SelectTemplate(Object item, DependencyObject container)
   at System.Windows.Controls.ContentPresenter.ChooseTemplate()
   at System.Windows.Controls.ContentPresenter.EnsureTemplate()
   at System.Windows.Controls.ContentPresenter.OnPreApplyTemplate()
   at System.Windows.FrameworkElement.ApplyTemplate()
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.UIElement.UpdateLayout()

Answer 1

My Colleague who had been working on this before me had added this line of code to the ViewModel without me noticing and this caused all the issues due to it returning before design time data had been loaded

    public CouponViewerViewModel()
    {

        if (DesignerProperties.IsInDesignMode == true)
        {
            return;
        }
        if (DesignerProperties.IsInDesignMode)
        {
            LoadDesignTimeData();
        }

    }

Apologies for a silly question in the end