[Client] Add temporary fix for crashes caused by custom GUI elements

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -908,8 +908,23 @@ namespace MWGui
                     window->onFrame(frameDuration);
         }
 
+        /*
+            Start of tes3mp change (major)
+
+            Custom GUI elements added by TES3MP often cause a crash here when their
+            mMainWidget becomes null, so a temporary fix has been added until a
+            more appropriate solution is researched
+        */
         if (!mCurrentModals.empty())
-            mCurrentModals.back()->onFrame(frameDuration);
+        {
+            if (mCurrentModals.back()->mMainWidget != 0)
+                mCurrentModals.back()->onFrame(frameDuration);
+            else
+                mCurrentModals.pop_back();
+        }
+        /*
+            End of tes3mp change (major)
+        */
 
         mKeyboardNavigation->onFrame();