Resolve crashes on exit with visible MyGUI widgets

CHANGELOG.md

@@ -108,6 +108,7 @@
     Bug #5869: Guards can initiate arrest dialogue behind locked doors
     Bug #5871: The console appears if you type the Russian letter "Ё" in the name of the enchantment
     Bug #5877: Effects appearing with empty icon
+    Bug #5899: Visible modal windows and dropdowns crashing game on exit
     Feature #390: 3rd person look "over the shoulder"
     Feature #832: OpenMW-CS: Handle deleted references
     Feature #1536: Show more information about level on menu

apps/openmw/mwgui/layout.cpp

@@ -35,6 +35,7 @@ namespace MWGui
 
     void Layout::shutdown()
     {
+        setVisible(false);
         MyGUI::Gui::getInstance().destroyWidget(mMainWidget);
         mListWindowRoot.clear();
     }

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -503,8 +503,6 @@ namespace MWGui
         {
             mStatsWatcher.reset();
 
-            mKeyboardNavigation.reset();
-
             MyGUI::LanguageManager::getInstance().eventRequestTag.clear();
             MyGUI::PointerManager::getInstance().eventChangeMousePointer.clear();
             MyGUI::InputManager::getInstance().eventChangeKeyFocus.clear();
@@ -523,6 +521,8 @@ namespace MWGui
             delete mCursorManager;
             delete mToolTips;
 
+            mKeyboardNavigation.reset();
+
             cleanupGarbage();
 
             mFontLoader.reset();