Fix null context 2116

[beto-rodriguez]

fixes #2116

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[Copilot]

Pull request overview

Fixes the NullReferenceException reported in #2116 by making text-paint change handling resilient to null during view/DataContext teardown, and adds a cross-platform sample + UI test scenario to cover it.

Changes:

• Prevent NRE in OnTextPaintPropertyChanged when the incoming paint is null.
• Add “NullContext” sample views (WPF/WinUI/Maui/Avalonia) plus a shared ViewModel to reproduce the binding/DataContext-clear scenario.
• Add a new shared UI test to validate charts survive a DataContext = null transition without crashing.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/SharedUITests/OtherTests.cs	Adds a UI test intended to reproduce the null-context crash.
src/skiasharp/_Shared/SourceGenChart.sgp.cs	Applies the null-safe access when setting PaintStyle for text paints.
samples/WinUISample/WinUISample/Samples/Test/NullContext/View.xaml.cs	Adds a WinUI test view with a helper to clear DataContext.
samples/WinUISample/WinUISample/Samples/Test/NullContext/View.xaml	Defines WinUI charts bound to paint properties affected by the bug.
samples/WPFSample/Test/NullContext/View.xaml.cs	Adds a WPF test view with a helper to clear DataContext.
samples/WPFSample/Test/NullContext/View.xaml	Defines WPF charts bound to paint properties affected by the bug.
samples/ViewModelsSamples/Test/NullContext/ViewModel.cs	Provides paint-bound properties to trigger text paint property changed callbacks.
samples/MauiSample/Test/NullContext/View.xaml.cs	Adds a MAUI test page with a helper to clear BindingContext.
samples/MauiSample/Test/NullContext/View.xaml	Defines MAUI charts bound to paint properties affected by the bug.
samples/AvaloniaSample/Test/NullContext/View.axaml.cs	Adds an Avalonia test view with a helper to clear DataContext and chart accessors.
samples/AvaloniaSample/Test/NullContext/View.axaml	Defines Avalonia charts bound to paint properties affected by the bug.

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