The latest change to Bind.cs results in System.ArgumentException being thrown when calling DependencyProperty.RegisterAttached method to register AtDesignTimeProperty. The property is declared as bool but a null is passed as a default value for it during registration. The fix would be to change the following:

    public static DependencyProperty AtDesignTimeProperty =
        DependencyProperty.RegisterAttached(
            "AtDesignTime",
            typeof(bool),
            typeof(Bind),
            new PropertyMetadata(null, AtDesignTimeChanged)
            );

to:

    public static DependencyProperty AtDesignTimeProperty =
        DependencyProperty.RegisterAttached(
            "AtDesignTime",
            typeof(bool),
            typeof(Bind),
            new PropertyMetadata(false, AtDesignTimeChanged)
            );