Added '_innerSerialDisposable?.Dispose()' call when _innerSerialDisposable changes. (#15998)

src/Avalonia.Base/Reactive/Operators/Switch.cs

@@ -54,6 +54,7 @@ internal sealed class Switch<TSource> : IObservable<TSource>
 
             var innerObserver = new InnerObserver(this, id);
 
+            _innerSerialDisposable?.Dispose();
             _innerSerialDisposable = innerObserver;
             innerObserver.Disposable = value.Subscribe(innerObserver);
         }