[karlmoriaty]

tl;dr

BrushToHexConverter casts its input straight to SolidColorBrush. In the XAML
designer that input can be the mock view model, so the cast throws and the design
surface fails with the error from #1411. I added a type check.

What

One line in src/MaterialDesignDemo.Shared/Converters/BrushToHexConverter.cs:
the explicit cast is replaced by a pattern match that returns null when the
input is not a brush.

Why

ColorTool.xaml binds the hex labels like this:

<TextBlock DataContext="{DynamicResource PrimaryHueMidBrush}"
           Text="{Binding Converter={StaticResource BrushToHexConverter}}" />

At runtime PrimaryHueMidBrush always resolves, so the converter always gets a
brush and nothing changes.

In the designer the lookup can fail. DataContext then falls back to the
inherited one, which is the mock object the designer creates for
d:DesignInstance Type=sharedDomain:ColorToolViewModel. The cast throws:

Unable to cast object of type 'Mocks.MaterialDesignDemo_ColorToolViewModel_...'
to type 'System.Windows.Media.SolidColorBrush'

That is #1411. With the type check the label renders empty in the designer
instead of breaking the design surface.

Scope

Type guard only. The design-time view model suggested in #1411 would also give
the designer real data, but that is a bigger change and I would rather keep
this one small.

Fixes #1411

Verification

• dotnet build src/MainDemo.Wpf/MaterialDesignDemo.csproj -f net8.0-windows: 0 warnings, 0 errors
• Runtime behaviour unchanged: every binding that uses this converter passes a SolidColorBrush

[karlmoriaty]

@Keboo the Pull Request Verification workflow is waiting on first-time contributor approval. Could you approve it when you get a chance? The change is the one-line cast replacement described above.