NRE in design mode with MahApps HamburgerMenu control

[Antony Male]

Discussed in #289

^{Originally posted by sm-g October 1, 2021}

Object reference not set to an instance of an object.
   at Stylet.Xaml.View.<>c.<.cctor>b__10_0(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
   at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
   at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
   at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp)
   at System.Windows.Data.BindingExpressionBase.Invalidate(Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.Activate(Object item)
   at System.Windows.Data.BindingExpression.AttachToContext(AttachAttempt attempt)
   at System.Windows.Data.BindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Run(Object arg)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

But everything work in runtime. Can I solve this issue?

(Hope this condensed xaml still reproduces the issue)

<UserControl x:Class="Foo"
             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:mah="http://metro.mahapps.com/winfx/xaml/controls"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:pages="clr-namespace:Foo.Pages"
             xmlns:s="https://github.com/canton7/Stylet"
             d:DesignHeight="450"
             d:DesignWidth="800"
             mc:Ignorable="d">
    <mah:HamburgerMenu 
                       Content="{Binding Pages/}"
                       ItemsSource="{Binding MenuItems}"
                       SelectedItem="{Binding MenuItems/}">

        <mah:HamburgerMenu.ContentTemplate>
            <DataTemplate DataType="{x:Type pages:PageViewModelBase}">
                        <ContentControl s:View.Model="{Binding}" />
            </DataTemplate>
        </mah:HamburgerMenu.ContentTemplate>
    </mah:HamburgerMenu>
</UserControl>
```</div>