Fix a crash when exit() already hides the window (Fixes #4148)

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -1729,9 +1729,10 @@ namespace MWGui
     {
         if (!mCurrentModals.empty())
         {
-            if (!mCurrentModals.back()->exit())
+            WindowModal* window = mCurrentModals.back();
+            if (!window->exit())
                 return;
-            mCurrentModals.back()->setVisible(false);
+            window->setVisible(false);
         }
     }