Result of IChangeToken.RegisterChangeCallback is nullable

src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs

@@ -36,7 +36,7 @@ public partial interface IChangeToken
     {
         bool ActiveChangeCallbacks { get; }
         bool HasChanged { get; }
-        System.IDisposable RegisterChangeCallback(System.Action<object?> callback, object? state);
+        System.IDisposable? RegisterChangeCallback(System.Action<object?> callback, object? state);
     }
     public readonly partial struct StringSegment : System.IEquatable<Microsoft.Extensions.Primitives.StringSegment>, System.IEquatable<string>
     {

src/libraries/Microsoft.Extensions.Primitives/src/IChangeToken.cs

@@ -28,6 +28,6 @@ public interface IChangeToken
         /// <param name="callback">The <see cref="Action{Object}"/> to invoke.</param>
         /// <param name="state">State to be passed into the callback.</param>
         /// <returns>An <see cref="IDisposable"/> that is used to unregister the callback.</returns>
-        IDisposable RegisterChangeCallback(Action<object?> callback, object? state);
+        IDisposable? RegisterChangeCallback(Action<object?> callback, object? state);
     }
 }