Add null checks and improve dialog window handling

Added early null checks for `hwnd` to prevent invalid processing.
Enhanced thread safety by locking `_dialogWindow` updates.
Documented dialog window state handling with comments for clarity.
Handled scenarios for dialog window movement, resizing, hiding,
destruction, and termination. Improved robustness and maintainability.

Author: Jack251970

Flow.Launcher.Infrastructure/DialogJump/DialogJump.cs

@@ -496,6 +496,8 @@ private static async void ForegroundChangeCallback(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             await _foregroundChangeLock.WaitAsync();
             try
             {
@@ -647,6 +649,8 @@ private static void LocationChangeCallback(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             // If the dialog window is moved, update the Dialog Jump window position
             var dialogWindowExist = false;
             lock (_dialogWindowLock)
@@ -672,6 +676,8 @@ private static void MoveSizeCallBack(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             // If the dialog window is moved or resized, update the Dialog Jump window position
             if (_dragMoveTimer != null)
             {
@@ -697,6 +703,8 @@ private static void DestroyChangeCallback(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             // If the dialog window is destroyed, set _dialogWindowHandle to null
             var dialogWindowExist = false;
             lock (_dialogWindowLock)
@@ -728,6 +736,8 @@ private static void HideChangeCallback(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             // If the dialog window is hidden, set _dialogWindowHandle to null
             var dialogWindowExist = false;
             lock (_dialogWindowLock)
@@ -759,6 +769,8 @@ private static void DialogEndChangeCallback(
             uint dwmsEventTime
         )
         {
+            if (hwnd.IsNull) return;
+
             // If the dialog window is ended, set _dialogWindowHandle to null
             var dialogWindowExist = false;
             lock (_dialogWindowLock)