[skip ci] App: print affected Python type if serializing via JSON fails

FemGui: add default implementations of __getstate__/__setstate__ to _ViewProviderFemResultMechanical

src/App/PropertyPythonObject.cpp

@@ -109,6 +109,8 @@ std::string PropertyPythonObject::toString() const
         repr = str.as_std_string("ascii");
     }
     catch (Py::Exception&) {
+        Py::String typestr(this->object.type().str());
+        Base::Console().Error("PropertyPythonObject::toString(): failed for %s\n", typestr.as_string().c_str());
         Base::PyException e; // extract the Python error text
         e.ReportException();
     }

src/Mod/Fem/femguiobjects/_ViewProviderFemResultMechanical.py

@@ -85,6 +85,12 @@ class _ViewProviderFemResultMechanical(ViewProviderFemConstraint.ViewProxy):
                 FreeCAD.Console.PrintError("Error in onDelete: {0} \n".format(err))
         return True
 
+    def __getstate__(self):
+        return None
+
+    def __setstate__(self, data):
+        return None
+
 
 class _TaskPanel:
     """