Catch case when Element is already added

GeoModelCore/GeoModelKernel/src/GeoMaterial.cxx

@@ -224,9 +224,10 @@ void GeoMaterial::add (const GeoElement* element, double fraction)
   // You can only add materials until you call "lock"...     
   if(m_locked) throw std::out_of_range ("Element added after material locked");
 
+  GeoIntrusivePtr<const GeoElement> elementPtr{element};
   auto e = std::find(m_elements.begin(),m_elements.end(),element);
   if (e==m_elements.end()) {
-    m_elements.push_back (element);
+    m_elements.push_back (elementPtr);
     m_fractions.push_back (fraction);
     element->ref ();
   }