DesktopGridEffect: Fix crash when closing the effect

If the first condition would match (for instance after moving a window),
`windowMove` would be `nullptr`, triggering the crash later.

By the looks of it, it was maybe a typo.

BUG: 445335
FIXED-IN: 5.24

CC: @nicolasfella

src/effects/desktopgrid/desktopgrid.cpp

@@ -445,7 +445,7 @@ void DesktopGridEffect::slotWindowClosed(EffectWindow* w)
         windowMove = nullptr;
     }
     if (isUsingPresentWindows()) {
-        const auto desktops = desktopList(windowMove);
+        const auto desktops = desktopList(w);
         for (const int i : desktops) {
             WindowMotionManager& manager = m_managers[w->screen()][i];
             manager.unmanage(w);