Qt: Fix closing log window via taskbar

src/duckstation-qt/logwindow.h

@@ -55,6 +55,7 @@ private:
   std::span<const char*> m_filter_names;
 
   bool m_attached_to_main_window = true;
+  bool m_destroying = false;
 };
 
 extern LogWindow* g_log_window;