This should fix the reported issue #21.

[nicholaskernan]

The groundBody does not have userData. Therefore it produces the traceback reported in issue #21.

[quozl]

Tested, does fix #21.

Reviewed. I'm fine with merging as is.

However, for perfection;

• [x] there's a change to activity_version, which wasn't mentioned in the commit message;
• [x] the commit message should have "groundBody did not have userData"; and,
• [x] there are a lot of changed lines because of indentation, which could be done instead by negating the condition and using continue, like this;

@@ -476,35 +476,36 @@ def get_world_model(self, ...

         bodylist = []
         for body in self.world.bodies:
+            if body == self.world.groundBody:
+                continue
             body.userData["saveid"] = save_id_index  # set temporary data
             ...

[nicholaskernan]

I modified the code and commit message based on your suggestions. Thanks.