[Client] Fix ListBox overlap crashes by removing ListBoxes properly

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -908,23 +908,8 @@ 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())
-        {
-            if (mCurrentModals.back()->mMainWidget != 0)
-                mCurrentModals.back()->onFrame(frameDuration);
-            else
-                mCurrentModals.pop_back();
-        }
-        /*
-            End of tes3mp change (major)
-        */
+            mCurrentModals.back()->onFrame(frameDuration);
 
         mKeyboardNavigation->onFrame();
 

apps/openmw/mwmp/GUIController.cpp

@@ -100,8 +100,13 @@ void mwmp::GUIController::setChatVisible(bool chatVisible)
 void mwmp::GUIController::showDialogList(const mwmp::BasePlayer::GUIMessageBox &guiMessageBox)
 {
     MWBase::WindowManager *windowManager = MWBase::Environment::get().getWindowManager();
-    windowManager->removeDialog(mListBox);
-    mListBox = nullptr;
+    
+    if (mListBox != NULL)
+    {
+        windowManager->removeDialog(mListBox);
+        windowManager->removeCurrentModal(mListBox);
+        mListBox = NULL;
+    }
 
     std::vector<std::string> list;