Add ValidationErrorTemplate into ComboBox

Fix issue #284

Author: l1pton17

MainDemo.Wpf/Domain/NotifyPropertyChangedExtension.cs

@@ -8,7 +8,7 @@ namespace MaterialDesignColors.WpfExample.Domain
     public static class NotifyPropertyChangedExtension
     {
         public static void MutateVerbose<TField>(this INotifyPropertyChanged instance, ref TField field, TField newValue, Action<PropertyChangedEventArgs> raise, [CallerMemberName] string propertyName = null)
-        {            
+        {
             if (EqualityComparer<TField>.Default.Equals(field, newValue)) return;
             field = newValue;
             raise?.Invoke(new PropertyChangedEventArgs(propertyName));            

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml

@@ -6,6 +6,7 @@
 
     <ResourceDictionary.MergedDictionaries>
         <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesignTheme.Shadows.xaml" />
+        <ResourceDictionary Source="MaterialDesignTheme.ValidationErrorTemplate.xaml" />
     </ResourceDictionary.MergedDictionaries>
 
     <converters:TextFieldHintVisibilityConverter x:Key="TextFieldHintVisibilityConverter" IsNotEmptyValue="Collapsed" />
@@ -694,6 +695,7 @@
         <Setter Property="ScrollViewer.CanContentScroll" Value="true" />
         <Setter Property="ScrollViewer.PanningMode" Value="Both" />
         <Setter Property="Stylus.IsFlicksEnabled" Value="False" />
+        <Setter Property="Validation.ErrorTemplate" Value="{StaticResource MaterialDesignValidationErrorTemplate}"/>
         <Setter Property="Template" Value="{StaticResource MaterialDesignComboBoxTemplate}" />
         <Style.Triggers>
             <Trigger Property="IsMouseOver" Value="true">