[GUI] Invalidate static dangling pointer (sofa-framework#5855)

Invalidate static pointer once it has been deleted

Author: alxbilger

Sofa/GUI/Common/src/sofa/gui/common/GUIManager.cpp

@@ -251,6 +251,7 @@ void GUIManager::closeGUI()
     if(currentGUI)
     {
         currentGUI->closeGUI();
+        currentGUI = nullptr;
     }
     else
     {