change the timing to remove PropertyChanged event (#498)

Author: takytank

Source/ReactiveProperty.Core/Internals/SimplePropertyObservable.cs

@@ -60,8 +60,8 @@ public Subscription(TSubject subject, Func<TSubject, TProperty> accessor, string
 
         private void PropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
-            if (_isDisposed) throw new InvalidOperationException();
             if (e.PropertyName != _propertyName && !string.IsNullOrEmpty(e.PropertyName)) return;
+            if (_isDisposed) throw new InvalidOperationException();
 
             try
             {
@@ -80,6 +80,7 @@ public void Dispose()
             if (_isDisposed) return;
 
             _isDisposed = true;
+            _subject.PropertyChanged -= PropertyChanged;
             if (_onError is false)
             {
                 try
@@ -92,7 +93,6 @@ public void Dispose()
                 }
             }
 
-            _subject.PropertyChanged -= PropertyChanged;
             _subject = default!;
             _observer = null!;
             _accessor = null!;