more safety options for non existing materials

bind/example.py

@@ -1,12 +1,10 @@
 
 from G4Calo import GeometryDescriptor, G4System
 
-cw = GeometryDescriptor()#the width of the calorimeter is 50 cm times 50 cm (also steerable, but I'd leave it)
-cw.addLayer(10,"G4_Si",True) #10 cm of Silicon, active
-cw.addLayer(1,"G4_Pb",False) #1 cm of Lead, passive
-cw.addLayer(10,"G4_Si",True, 3) #10 cm of Silicon, active, 3x3 segmentation
+cw = GeometryDescriptor()
+for _ in range(25):
+    cw.addLayer(0.5, "G4Pb", False)
+    cw.addLayer(1.,"G4_POLYSTYRENE",True,1)
 
 #directly use G4System as a global singleton
 G4System.init(cw)
-
-G4System.displayEvent()

src/DetectorConstruction.cc

@@ -129,7 +129,10 @@ void DetectorConstruction::DefineMaterials()
 nistManager->FindOrBuildMaterial("G4_AIR");
   auto cwLayers = cw->getLayers();
   for(auto layer : cwLayers){
-      nistManager->FindOrBuildMaterial(layer.material);
+
+      auto ret = nistManager->FindOrBuildMaterial(layer.material);
+      if(ret == nullptr)
+          throw std::runtime_error("material not found: " + layer.material);
   }
 
   // Print materials