NullReferenceException without explicit font specification

Running my app on Linux Mate on a Raspberry gives me the following error

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.Skia.TypefaceCache.GetTypeface(String name, FontKey key)
   at Avalonia.Skia.FormattedTextImpl..ctor(String text, Typeface typeface, TextAlignment textAlignment, TextWrapping wrapping, Size constraint, IReadOnlyList`1 spans)
   at Avalonia.Skia.PlatformRenderInterface.CreateFormattedText(String text, Typeface typeface, TextAlignment textAlignment, TextWrapping wrapping, Size constraint, IReadOnlyList`1 spans)
   at Avalonia.Controls.TextBlock.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Controls.Grid.<>c__DisplayClass25_0.<MeasureOverride>g__MeasureOnce|2(Control child, Size size)
   at Avalonia.Controls.Grid.<>c__DisplayClass25_0.<MeasureOverride>b__0(Control child)
   at Avalonia.Controls.Utils.GridLayout.AppendMeasureConventions[T](IDictionary`2 source, Func`2 getDesiredLength)
   at Avalonia.Controls.Grid.MeasureOverride(Size constraint)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Controls.Grid.<>c__DisplayClass25_0.<MeasureOverride>g__MeasureOnce|2(Control child, Size size)
   at Avalonia.Controls.Grid.<>c__DisplayClass25_0.<MeasureOverride>b__0(Control child)
   at Avalonia.Controls.Utils.GridLayout.AppendMeasureConventions[T](IDictionary`2 source, Func`2 getDesiredLength)
   at Avalonia.Controls.Grid.MeasureOverride(Size constraint)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutHelper.MeasureChild(ILayoutable control, Size availableSize, Thickness padding)
   at Avalonia.Controls.Presenters.ContentPresenter.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutHelper.MeasureChild(ILayoutable control, Size availableSize, Thickness padding)
   at Avalonia.Controls.Primitives.AdornerDecorator.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutHelper.MeasureChild(ILayoutable control, Size availableSize, Thickness padding)
   at Avalonia.Controls.Border.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureOverride(Size availableSize)
   at Avalonia.Controls.Window.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutManager.Measure(ILayoutable control)
   at Avalonia.Layout.LayoutManager.ExecuteInitialLayoutPass(ILayoutRoot root)
   at Avalonia.Controls.Window.Show()
   at Avalonia.Application.Run(Window mainWindow)
   at Avalonia.Controls.AppBuilderBase`1.Start[TMainWindow](Func`1 dataContextProvider)
   at MyApp.Program.Main(String[] args) in C:\MyApp\Program.cs:line 11
Aborted

The error is resolved by setting the FontFamily property on my main window. In this case I have to set it to Ubuntu. However, when I run the app on Linux Raspian it fails again with the same error. Setting the FontFamily to Piboto solves it. Actually all I want is that the app uses the default desktop font, whatever that is, without specifying it explicitly. Is this is problem in Skia or Avalonia or are we as application developers to required to fulfill any prerequisites?

[Gillibald]

Will change the logic to use SKTypeface.FromFamilyName(null) when null is returned.

@mattleibow Thanks for making this clear.

[mattleibow]

I had a look and it appears that in some cases, the fontconfig font manager will return null. I don’t think there was ever a guarantee that if you request a font it would return a system default.

See this line: https://github.com/google/skia/blob/chrome/m68/src/ports/SkFontMgr_fontconfig.cpp#L894

In the next (m68) release, I added a SKTypeface.Default property that can be used and should never be null as per the comment in the source: https://github.com/google/skia/blob/chrome/m68/include/core/SkTypeface.h#L90-L91

In the meantime, a fallback is to pass null to SKTypeface.FromFamilyName(null). The source for that reaches here: https://github.com/google/skia/blob/chrome/m68/src/core/SkTypeface.cpp#L135-L144 and the GetDefaultTypeface reference is the same used in SkTypeface::MakeDefault(): https://github.com/google/skia/blob/chrome/m68/src/core/SkTypeface.cpp#L97-L112