Fix Carousel layout crashes from missing null/bounds checks in AdaptiveCarouselRenderer

[karan68]

Summary

Fixes multiple crash paths in AdaptiveCarouselRenderer that cause STOWED_EXCEPTION (0x88000fa8) during XAML layout pass when rendering Carousel elements.

Affected apps: WidgetBoard (77.6%), lockapp (3.9%)
Affected SDK: AdaptiveCards.Rendering.Uwp 3.2.x, AdaptiveCards.Rendering.WinUI3 2.2.3-beta

Root Cause

AdaptiveCarouselRenderer.cpp has several unguarded code paths that throw during XAML layout/rendering:

Bug 1 — SelectionChanged handler crashes with SelectedIndex = -1

When carouselUI.Items().Append() is called during rendering, the FlipView fires SelectionChanged before any item is selected (SelectedIndex() == -1). The handler calls pipsPager.SelectedPageIndex(-1) which throws InvalidOperationException because PipsPager does not accept negative indices.

Bug 2 — SetFlipViewMaxHeight calls GetAt() without bounds check

flipView.SelectedIndex() can return -1 (no selection). GetAt(-1) throws E_BOUNDS. This function is called from a LayoutUpdated event handler — outside the try-catch in Render() — so the exception propagates unhandled and becomes a stowed exception that crashes the host process.

Bug 3 — SetFlipViewMaxHeight calls .Measure() on unchecked try_as result

try_as<FrameworkElement>() can return nullptr. The code calls .Measure() on the result without a null check → null pointer dereference.

Bug 4 — try_as<FrameworkElement>() chained directly into .LayoutUpdated()

In the render loop, carouselPageUI.try_as<FrameworkElement>().LayoutUpdated(...) chains the method call without checking if try_as returned null.

Bug 5 — InitialPage index used without bounds validation

carousel.InitialPage().GetUInt32() is passed directly to SelectedIndex() without checking it's within Items().Size().

Fix

• SelectionChanged handler: Early return when SelectedIndex() < 0
• SetFlipViewMaxHeight: Validate selectIndex >= 0 && < Items().Size() before GetAt(). Null-check try_as result before .Measure().
• Render loop: Store try_as result and guard .LayoutUpdated() call with null check.
• InitialPage: Validate index against carouselUI.Items().Size() before setting.

Repro

1. Build and run source/uwp/winui3/SimpleVisualizer/SimpleVisualizer.sln
2. Click "Show Adaptive Card" —> loads ClipchampCarousel.json
3. Before fix: System.InvalidOperationException: 'Operation is not valid due to the current state of the object.' during RenderAdaptiveCard()
4. After fix: Carousel renders and displays without crash

Files Changed

• source/uwp/SharedRenderer/lib/AdaptiveCarouselRenderer.cpp —> All 5 fixes
• source/uwp/winui3/SimpleVisualizer/ClipchampCarousel.json —> Test template (Clipchamp-style Carousel with empty Container)
• source/uwp/winui3/SimpleVisualizer/MainWindow.xaml.cs —> Use AppContext.BaseDirectory instead of Package.Current.InstalledLocation.Path