Allow the calculator to return None as a result

Author: mesonepigreco

Modules/Cluster.py

@@ -820,7 +820,9 @@ def read_results(self, calc, label):
         calc.read_results()
         #calc.structure.save_scf("thread_{}_justreaded_{}.scf".format(threading.get_native_id(), label))
         results = copy.deepcopy(calc.results)
-        results["structure"] = calc.structure.copy()  # Report also the structure to check consistency
+
+        if results is not None:
+            results["structure"] = calc.structure.copy()  # Report also the structure to check consistency
 
         return results