+ fix crash in DocumentObserverPython if a property has no name (beca…

…use it's not part of an object)

src/App/DocumentObserverPython.cpp

@@ -239,9 +239,13 @@ void DocumentObserverPython::slotChangedObject(const App::DocumentObject& Obj,
             Py::Callable method(this->inst.getAttr(std::string("slotChangedObject")));
             Py::Tuple args(2);
             args.setItem(0, Py::Object(const_cast<App::DocumentObject&>(Obj).getPyObject(), true));
-            std::string prop_name = Obj.getPropertyName(&Prop);
-            args.setItem(1, Py::String(prop_name));
-            method.apply(args);
+            // If a property is touched but not part of a document object then its name is null.
+            // In this case the slot function must not be called.
+            const char* prop_name = Obj.getPropertyName(&Prop);
+            if (prop_name) {
+                args.setItem(1, Py::String(prop_name));
+                method.apply(args);
+            }
         }
     }
     catch (Py::Exception&) {