fixed a few little things

include/G4System.hh

@@ -28,6 +28,7 @@ G4System(bool Gui=false):gui(Gui){};
     delete runManager;
     delete ui;
 }
+
 };
 
 
@@ -43,15 +44,22 @@ void applyUICommand(const std::string& command){
     UImanager->ApplyCommand(command);
 }
 
-ConstructionWrapper cw;
-
 void displayEvent()const{}; //just a placeholder, this will be implemented in python
 
 void printMaterial(const std::string& name)const;
 
+ConstructionWrapper& getConstructionWrapper(){
+    check();
+    if ( assigned_cw == nullptr ){
+        throw std::runtime_error("ConstructionWrapper not assigned");
+    }
+    return *assigned_cw;
+}
+
 private:
 
 void check()const;
+ConstructionWrapper * assigned_cw=nullptr;
 
 bool gui;