Fixed InAppNotification not dismissing programmatically after cancelling closing event (#4424)

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Fixes #4423 

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## PR Type

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Bugfix
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## What is the current behavior?
When the Closing event is cancelled, subsequent calls to Dismiss() no longer work as expected.

## What is the new behavior?
Fixed an issue where InAppNotification couldn't be dismissed programmatically after cancelling the closing event.

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Author: msftbot[bot]

Microsoft.Toolkit.Uwp.UI.Controls.Core/InAppNotification/InAppNotification.cs

@@ -184,6 +184,14 @@ private void Dismiss(InAppNotificationDismissKind dismissKind, bool dismissAll =
                 return;
             }
 
+            var closingEventArgs = new InAppNotificationClosingEventArgs(dismissKind);
+            Closing?.Invoke(this, closingEventArgs);
+
+            if (closingEventArgs.Cancel)
+            {
+                return;
+            }
+
             _dismissTimer.Stop();
 
             // Dismiss all if requested
@@ -212,14 +220,6 @@ private void Dismiss(InAppNotificationDismissKind dismissKind, bool dismissAll =
                 return;
             }
 
-            var closingEventArgs = new InAppNotificationClosingEventArgs(dismissKind);
-            Closing?.Invoke(this, closingEventArgs);
-
-            if (closingEventArgs.Cancel)
-            {
-                return;
-            }
-
             var result = VisualStateManager.GoToState(this, StateContentCollapsed, true);
             if (!result)
             {