Fix NullReferenceException on dispose finalization on NewPipsPager

Author: neon-nyan

CollapseLauncher/Classes/Extension/UIElementExtensions.UnsafeAccessorExtensions.cs

@@ -1,6 +1,7 @@
 using Microsoft.UI.Input;
 using Microsoft.UI.Xaml;
 using System.Runtime.CompilerServices;
+using WinRT;
 
 #nullable enable
 namespace CollapseLauncher.Extension
@@ -25,5 +26,20 @@ internal static T WithCursor<T>(this T element, InputCursor inputCursor) where T
             element.SetCursor(inputCursor);
             return element;
         }
+
+        [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_disposedFlags")]
+        private static extern ref int GetObjectReferenceDisposeFlags(this IObjectReference obj);
+
+        /// <summary>
+        /// Check whether a WinRT object has been disposed.
+        /// </summary>
+        /// <returns><see langword="true"/> if object is already disposed. Otherwise, <see langword="false"/>.</returns>
+        internal static bool IsObjectDisposed(this IWinRTObject winRtObject)
+        {
+            IObjectReference reference = winRtObject.NativeObject;
+            ref int disposeFlags = ref reference.GetObjectReferenceDisposeFlags();
+
+            return disposeFlags > 0;
+        }
     }
 }

CollapseLauncher/XAMLs/Theme/CustomControls/NewPipsPager/NewPipsPager.Templates.cs

@@ -80,8 +80,15 @@ private void ApplyNavigationButtonEvents()
 
     private void UnapplyNavigationButtonEvents()
     {
-        _previousPageButton.Click -= PreviousPageButton_OnClick;
-        _nextPageButton.Click     -= NextPageButton_OnClick;
+        if (!_previousPageButton.IsObjectDisposed())
+        {
+            _previousPageButton.Click -= PreviousPageButton_OnClick;
+        }
+
+        if (!_nextPageButton.IsObjectDisposed())
+        {
+            _nextPageButton.Click -= NextPageButton_OnClick;
+        }
     }
 
     private void ApplyKeyPressEvents()
@@ -104,7 +111,10 @@ private void ApplyItemsRepeaterEvents()
 
     private void UnapplyItemsRepeaterEvents()
     {
-        _pipsPagerItemsRepeater.ElementPrepared -= ItemsRepeater_ElementPrepared;
+        if (!_pipsPagerItemsRepeater.IsObjectDisposed())
+        {
+            _pipsPagerItemsRepeater.ElementPrepared -= ItemsRepeater_ElementPrepared;
+        }
     }
 
     #endregion