set m_Window to nullptr when window is destroyed

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/Window/OpenGLWindow.cpp

@@ -171,7 +171,7 @@ namespace Engine {
     void OpenGLWindow::Shutdown() {
         if (m_Window) {
             glfwDestroyWindow(m_Window);
-            // m_Window = nullptr;
+            m_Window = nullptr;
         }
         glfwTerminate();
     }