NullReferenceException for PART_HeaderButton in WPF Calendar - wpf

Question

I have a custom control template for my CalendarItem, and it works great except that when you click the month (<Button x:Name="PART_HeaderButton">) the app crashes with:
System.NullReferenceException: 'Object reference not set to an instance of an object.'
Here is most of the error detail.
System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=PresentationFramework
StackTrace:
at System.Windows.Controls.Primitives.CalendarItem.<GetCalendarButtons>d__56.MoveNext()
at System.Windows.Controls.Primitives.CalendarItem.GetCalendarButton(DateTime date, CalendarMode mode)
at System.Windows.Controls.Primitives.CalendarItem.FocusDate(DateTime date)
at System.Windows.Controls.Primitives.CalendarItem.HeaderButton_Click(Object sender, RoutedEventArgs e)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.Controls.Primitives.ButtonBase.OnClick()
at System.Windows.Controls.Button.OnClick()
I don't know how to deal with MoveNext or GetCalendarButton or anything, but the thing is, I don't care. I don't need the month name to be clickable. If a fix involved actually fixing it to make it work, that's fine, but what I really care about is that my app doesn't crash! I don't need a year mode or decade mode anyway.
Obviously, the simple answer is to set the button to IsEnabled="False", however, that's not working for some reason! I have <Button x:Name="PART_HeaderButton" IsEnabled="False"/> and the button is NOT disabled. Sometimes, if I delete the IsEnabled, and then put it back, the button becomes disabled, but when I move to another view and back, or restart the app, it's back to a normal button. Setting Command={x:Null} also doesn't stop the button from trying to execute (and hence crashing).
Since I'm at a loss for what the heck is going on, and I don't know where to focus on, here is my whole damn calendar:
<Calendar.CalendarItemStyle>
<Style TargetType="{x:Type CalendarItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CalendarItem}">
<ControlTemplate.Resources>
<!--DAY OF WEEK-->
<DataTemplate x:Key="{x:Static CalendarItem.DayTitleTemplateResourceKey}">
<TextBlock FontSize="12" TextAlignment="Center" Text="{Binding}" VerticalAlignment="Center"/>
</DataTemplate>
</ControlTemplate.Resources>
<!--Entire calendar-->
<Border Padding="8 0" Background="{StaticResource BackgroundBlack}" x:Name="itemBorder">
<Grid Name="PART_Root">
<Grid.RowDefinitions>
<!--Month and month nav row-->
<RowDefinition Height="{StaticResource monthHeaderHeight}"/>
<!--Month view (days grid)-->
<RowDefinition/>
</Grid.RowDefinitions>
<!--TOP ROW (Month)-->
<DockPanel Grid.Column="0" Grid.Row="0" VerticalAlignment="Center" LastChildFill="False">
<DockPanel.Resources>
<Style TargetType="Button" BasedOn="{StaticResource PlainTextButton}"/>
<Style TargetType="Button" BasedOn="{StaticResource PlainTextButton}" x:Key="arrowStyle">
<Setter Property="Height" Value="14"/>
<Setter Property="Width" Value="36"/>
</Style>
</DockPanel.Resources>
<!--MONTH NAVIGATION-->
<!--Unclear why they have to be in reverse order.-->
<Button x:Name="PART_NextButton" DockPanel.Dock="Right" Style="{StaticResource arrowStyle}">
<Image Source="/Resources/Images/Right.png"/>
</Button>
<Button x:Name="PART_PreviousButton" DockPanel.Dock="Right" Style="{StaticResource arrowStyle}">
<Image Source="/Resources/Images/Left.png"/>
</Button>
<!--CURRENT MONTH-->
<!--TODO: Make sure this is NOT CLICKABLE because clicking crashes (IsEnabled="False" doesn't seem to fix it)-->
<Button x:Name="PART_HeaderButton" IsEnabled="False"/>
</DockPanel>
<Grid Grid.Column="0" Grid.Row="1" VerticalAlignment="Center" HorizontalAlignment="Center">
<Grid x:Name="PART_MonthView">
<Grid.RowDefinitions>
<!--Weeks (first row is Days of the Week)-->
<RowDefinition Height="{StaticResource dayOfWeekHeight}"/>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<!--Days-->
<ColumnDefinition/>
<ColumnDefinition/>
<ColumnDefinition/>
<ColumnDefinition/>
<ColumnDefinition/>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
</Grid>
</Grid>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Calendar.CalendarItemStyle>

Answer 1

First, I always recommend looking at the .NET Reference Source to understand what is going on inside the control.
https://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Controls/Primitives/CalendarItem.cs
The reason why IsEnabled isn't working is that CalendarItem automatically sets IsEnabled on the button internally.
You might want to consider one of these options:
Set IsHitTestVisible="False" on the button instead to prevent it from being clickable
Don't include the button at all and display your own month label using the DisplayDate property and a value converter to get the month name
I hope this is helpful.