converter fix

PacticeSolution/BindingValidationRuleSample/Converter/ButtonTextBrush.cs

@@ -15,10 +15,10 @@ namespace BindingValidationRuleSample.Converter
         public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
         {
             if (targetType != typeof(Brush))
-                return null;
+                return Brushes.Black;
 
             if (value == null)
-                return null;
+                return Brushes.Black;
 
             int age = int.Parse(value.ToString());
             return ValueColor(age);