fix crash when removing from document and accessing Python based feature

src/App/FeaturePython.h

@@ -172,8 +172,12 @@ public:
     /** @name Property serialization */
     //@{
     void Save (Base::Writer &writer) const {
-        writer.ObjectName = this->getNameInDocument();
-        props->Save(writer);
+        const char* objname = this->getNameInDocument();
+        // if null then it's not part of the document
+        if (objname) {
+            writer.ObjectName = objname;
+            props->Save(writer);
+        }
     }
     void Restore(Base::XMLReader &reader) {
         props->Restore(reader);