Change Nullability annotation on ISelectionService.PrimarySelection (#81254)

* Change Nullability on ISelectionService.PrimarySelection

* Add nullability to ref ISelectionService interface

Author: elachlan

src/libraries/System.ComponentModel.TypeConverter/ref/System.ComponentModel.TypeConverter.cs

@@ -1943,7 +1943,7 @@ public partial interface IRootDesigner : System.ComponentModel.Design.IDesigner,
     }
     public partial interface ISelectionService
     {
-        object PrimarySelection { get; }
+        object? PrimarySelection { get; }
         int SelectionCount { get; }
         event System.EventHandler SelectionChanged;
         event System.EventHandler SelectionChanging;

src/libraries/System.ComponentModel.TypeConverter/src/System/ComponentModel/Design/ISelectionService.cs

@@ -13,7 +13,7 @@ public interface ISelectionService
         /// <summary>
         /// Gets the object that is currently the primary selection.
         /// </summary>
-        object PrimarySelection { get; }
+        object? PrimarySelection { get; }
 
         /// <summary>
         /// Gets the count of selected objects.