After upgrading from Telerik UI for .NET MAUI 13.0.0 → 13.2.0, a NullReferenceException is thrown inside SchedulerAgendaView.Init() on Windows (WinUI3) whenever a RadScheduler with an AgendaViewDefinition is used. The scheduler never renders and the app receives an unhandled exception.

Steps to Reproduce:

1. Create a .NET MAUI app targeting Windows (WinUI3)
2. Add a ContentPage (or any page) with a RadScheduler in XAML, including at least one AgendaViewDefinition in ViewDefinitions:

<telerik:RadScheduler AppointmentsSource="{Binding Appointments}">
    <telerik:RadScheduler.ViewDefinitions>
        <telerik:AgendaViewDefinition />
        <telerik:DayViewDefinition />
    </telerik:RadScheduler.ViewDefinitions>
</telerik:RadScheduler>

3. Navigate to that page
4. Observe the unhandled exception

Expected Behavior:

The scheduler renders correctly with the AgendaViewDefinition, as it did in 13.0.0.

Actual Behavior:

An unhandled NullReferenceException is thrown:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.Maui.Controls.Scheduler.SchedulerAgendaView.Init()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__124_0(Object state)
   at Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.<>c__DisplayClass2_0.<Post>b__0()

Root Cause Analysis (via ILSpy decompilation):

Comparing the decompiled assemblies of 13.0.0 and 13.2.0 reveals a new override in RadScheduler introduced in 13.2.0:

// NEW in 13.2.0 — not present in 13.0.0:
protected override void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
    base.OnPropertyChanged(propertyName);
    if (propertyName == "Parent" && Parent != null && !IsLoaded)
        UpdateActiveViewDefinition();
}

This causes the following call chain to be triggered during InitializeComponent() in the page constructor — before the page is attached to a window:

1. XAML parser adds RadScheduler to the page → Parent property changes
2. OnPropertyChanged("Parent") fires → UpdateActiveViewDefinition() is called
3. → ActiveViewDefinition = ViewDefinitions[0] (the AgendaViewDefinition)
4. → OnActiveViewDefinitionChanged() → content.Rebuild() (template already applied on WinUI3)
5. → SchedulerAgendaView.Model setter → Init() is called as async void
6. Inside Init(), after the first await, the continuation runs and DataBindingComplete fires
7. The completeHandler inside Init() calls:

((BindableObject)this).Dispatcher.Dispatch(async delegate { ... });

8. Dispatcher is null because the SchedulerAgendaView is not yet attached to a window → NullReferenceException

The exception is captured by the async void state machine and re-thrown via SynchronizationContext.ThrowAsync → DispatcherQueueSynchronizationContext.Post, which matches the observed stack trace exactly.

Why it worked in 13.0.0: UpdateActiveViewDefinition() was never called from OnPropertyChanged. It was only invoked after IsLoaded = true, at which point Dispatcher is guaranteed to be non-null.

Why the condition !IsLoaded is insufficient: Parent != null does not imply a window is present. During InitializeComponent(), the element has a parent in the logical tree but is not yet attached to any Window, making Dispatcher null on BindableObject.

Workaround:

Do not declare ViewDefinitions in XAML. Instead, add them programmatically in the page's Loaded event handler, at which point Dispatcher is guaranteed to be available:

private void OnPageLoaded(object sender, EventArgs e)
{
    if (scheduler.ViewDefinitions.Count == 0)
    {
        scheduler.ViewDefinitions.Add(new AgendaViewDefinition());
        scheduler.ViewDefinitions.Add(new DayViewDefinition());
        scheduler.ViewDefinitions.Add(new WeekViewDefinition());
        scheduler.ViewDefinitions.Add(new MonthViewDefinition());
    }
}

Suggested Fix:

Either:

Option A — Guard Dispatcher usage inside SchedulerAgendaView.Init():

// In completeHandler, before calling Dispatch:
if (((BindableObject)this).Dispatcher is { } dispatcher)
    dispatcher.Dispatch(...);

Option B — Guard UpdateActiveViewDefinition() in RadScheduler.OnPropertyChanged to only run when content (the internal RadSchedulerContent) has already been set (i.e., template was applied):

if (propertyName == "Parent" && Parent != null && !IsLoaded && content != null)
    UpdateActiveViewDefinition();

Option B is more conservative and closer to the original 13.0.0 behavior, since content being non-null implies the template has been applied and Dispatcher is available.

Environment:

After upgrading to .NET MAUI 10.0.60, the app crashes on Windows with an unhandled exception when RadCollectionView renders items that use a ControlTemplate containing a RadBorder.

Exception:

System.Exception: Keine installierten Komponenten gefunden. (REGDB_E_CLASSNOTREG)
  at ABI.System.Collections.Generic.IListMethods`2.AppendDynamic(...)
  at Microsoft.UI.Xaml.Controls.UIElementCollection.Add(UIElement item)
  at Telerik.Maui.RadBorderExtensions.UpdateBorderContent(Border nativeBorder, IRadBorder border)
  at Telerik.Maui.Handlers.RadBorderHandler.MapContent(...)
  at Telerik.Maui.Controls.RadCollectionViewItemView.OnStyleChanged()
  ...

Root Cause:

MAUI PR #30047 introduced a call to view.Handler?.DisconnectHandler() inside ContentViewHandler.UpdateContent (Windows-only) before re-adding the content to the native panel. This disconnects and invalidates Telerik's RadBorderHandler WinRT object references. When RadBorderExtensions.UpdateBorderContent subsequently calls UIElementCollection.Add, the WinRT interop throws the COM error.

Steps to reproduce:

1. Use MAUI 10.0.60 or later on Windows
2. Place a RadCollectionView with an ItemTemplate or ControlTemplate that contains a RadBorder
3. Run the app and display the collection

Expected: Collection renders correctly.
Actual: App crashes with REGDB_E_CLASSNOTREG.

Versions:

• .NET MAUI: 10.0.60
• Telerik UI for MAUI: 13.2.0 (also reproduced on 13.0.0)
• Platform: Windows (WinUI 3)

Workaround:
Replacing the default ContentViewHandler mapping with a custom implementation that calls platformView.RemoveFromParent() on the native view instead of DisconnectHandler() on the virtual view avoids the crash.

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