fixed assign problems probably

src/G4System.cc

@@ -14,6 +14,9 @@ void G4System::init(GeometryDescriptor &CW){
   if(CW.isAssigned()){
     throw std::runtime_error("GeometryDescriptor already assigned");
   }
+  if(CW.isEmpty()){
+    throw std::runtime_error("GeometryDescriptor is empty");
+  }
   if(assigned_cw != nullptr){
     //reassign
     assigned_cw->unAssign();