#1798 Fix binding checkbox with three state.

src/Avalonia.Controls/Primitives/ToggleButton.cs

@@ -14,7 +14,7 @@ namespace Avalonia.Controls.Primitives
                 nameof(IsChecked),
                 o => o.IsChecked,
                 (o, v) => o.IsChecked = v,
-                unsetValue: false,
+                unsetValue: null,
                 defaultBindingMode: BindingMode.TwoWay);
 
         public static readonly StyledProperty<bool> IsThreeStateProperty =