extra guards in converter fixes #462

Author: ButchersBoy

MaterialDesignThemes.Wpf/Converters/MathMultipleConverter.cs

@@ -15,24 +15,25 @@ public sealed class MathMultipleConverter : IMultiValueConverter
 
         public object Convert(object[] value, Type targetType, object parameter, CultureInfo culture)
         {
+            if (value == null || value.Length < 2 || value[0] == null || value[1] == null) return Binding.DoNothing;
+
             double value1, value2;
-            if (Double.TryParse(value[0].ToString(), out value1) && Double.TryParse(value[1].ToString(), out value2))
+            if (!double.TryParse(value[0].ToString(), out value1) || !double.TryParse(value[1].ToString(), out value2))
+                return Binding.DoNothing;
+
+            switch (Operation)
             {
-                switch (Operation)
-                {
-                    default:
-                    case MathOperation.Add:
-                        return value1 + value2;
-                    case MathOperation.Divide:
-                        return value1 / value2;
-                    case MathOperation.Multiply:
-                        return value1 * value2;
-                    case MathOperation.Subtract:
-                        return value1 - value2;
-                }
+                default:
+                // (case MathOperation.Add:)
+                    return value1 + value2;
+                case MathOperation.Divide:
+                    return value1 / value2;
+                case MathOperation.Multiply:
+                    return value1 * value2;
+                case MathOperation.Subtract:
+                    return value1 - value2;
             }
-
-            return Binding.DoNothing;
+            
         }
 
         public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)