fixes(DataGrid): Warning CS0649 Field 'DataGridPathGroupDescription._valueConverter' is never assigned to, and will always have its default value null

src/Avalonia.Controls.DataGrid/Collections/DataGridGroupDescription.cs

@@ -83,8 +83,9 @@ namespace Avalonia.Collections
             if (key == null)
                 key = item;
 
-            if (_valueConverter != null)
-                key = _valueConverter.Convert(key, typeof(object), level, culture);
+            var valueConverter = ValueConverter;
+            if (valueConverter != null)
+                key = valueConverter.Convert(key, typeof(object), level, culture);
 
             return key;
         }
@@ -99,6 +100,8 @@ namespace Avalonia.Collections
         }
         public override string PropertyName => _propertyPath;
 
+        public IValueConverter ValueConverter { get => _valueConverter; set => _valueConverter = value; }
+
         private Type GetPropertyType(object o)
         {
             return o.GetType().GetNestedPropertyType(_propertyPath);