FEM: base python object, add missing getstate method

src/Mod/Fem/femobjects/FemConstraint.py

@@ -37,10 +37,15 @@ class Proxy(object):
         # self.Object = obj  # keep a ref to the DocObj for nonGui usage
         obj.Proxy = self  # link between App::DocumentObject to this object
 
-    # a few objects had this method in their class before the move to this base class
-    # these objects will give a setAttr failed error on document loading without this method
+    # they are needed, see https://forum.freecadweb.org/viewtopic.php?f=18&t=44021
+    def __getstate__(self):
+        return None
+
     def __setstate__(self, state):
+        # a few FEM objects had this attribut assignment before the move to this Python base obj class
+        # these objects will give a setAttr failed error on document loading without this assignment
         if state:
             self.Type = state
+        return None
 
 ##  @}