Update NullableBoolConverter.cs (#8388)

* Update NullableBoolConverter.cs

* Update src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/NullableBoolConverter.cs

Co-authored-by: ThomasGoulet73 <[email protected]>

---------

Co-authored-by: ThomasGoulet73 <[email protected]>

Author: wstaelens

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/NullableBoolConverter.cs

@@ -57,15 +57,10 @@ public override bool GetStandardValuesExclusive(ITypeDescriptorContext context)
         /// <returns>TypeConverter.StandardValuesCollection</returns>
         public override TypeConverter.StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
         {
-            if (_standardValues == null)
+            return _standardValues ??= new TypeConverter.StandardValuesCollection(new Nullable<bool>[]
             {
-                ArrayList list1 = new ArrayList(3);
-                list1.Add((bool?)true);
-                list1.Add((bool?)false);
-                list1.Add((bool?)null);
-                _standardValues = new TypeConverter.StandardValuesCollection(list1.ToArray());
-            }
-            return _standardValues;
+                true, false, null
+            });
         }
 
         /// <summary>