address review feedback

proto/marshal/collections/repeated.py

@@ -105,7 +105,16 @@ def _pb_type(self):
             return type(self.pb[0])
 
         # We have no members in the list, so we get the type from the attributes.
-        return self.pb._message_descriptor._concrete_class
+        if hasattr(self.pb, "_message_descriptor") and hasattr(
+            self.pb._message_descriptor, "_concrete_class"
+        ):
+            return self.pb._message_descriptor._concrete_class
+
+        # Fallback logic in case attributes are not available
+        # In order to get the type, we create a throw-away copy and add a
+        # blank member to it.
+        canary = copy.deepcopy(self.pb).add()
+        return type(canary)
 
     def __eq__(self, other):
         if super().__eq__(other):