Add IsAlive property to IRef<T> (#20593)

src/Avalonia.Base/Media/Imaging/Bitmap.cs

@@ -291,9 +291,7 @@ namespace Avalonia.Media.Imaging
         {
             get
             {
-                // TODO12: We should probably make PlatformImpl to be nullable or make it possible to check
-                // and fix IRef<T> in general (right now Item is not nullable while it internally is)
-                if (PlatformImpl.Item == null!)
+                if (!PlatformImpl.IsAlive)
                     return null;
                 return PlatformImpl;
             }

src/Avalonia.Base/Utilities/Ref.cs

@@ -28,6 +28,10 @@ namespace Avalonia.Utilities
         /// <returns>A reference to the value as the new type but sharing the refcount.</returns>
         IRef<TResult> CloneAs<TResult>() where TResult : class;
 
+        /// <summary>
+        /// Gets whether the reference still tracks a valid item.
+        /// </summary>
+        bool IsAlive { get; }
 
         /// <summary>
         /// The current refcount of the object tracked in this reference. For debugging/unit test use only.
@@ -172,6 +176,8 @@ namespace Avalonia.Utilities
                 return new Ref<TResult>(item, counter);
             }
 
+            public bool IsAlive => _item is not null;
+
             public int RefCount => _counter?.RefCount ?? throw new ObjectDisposedException("Ref<" + typeof(T) + ">");
         }
     }