The Object reference not set to an instance of an object error is a common Windows issue from Microsoft Visual Studio. This error occurs when a Visual Studio object is absent, null, or inaccessible.

We show you how to fix Object reference not set to an instance of an object error prompt which you may see in Microsoft Visual Studio.

The first line reads Object reference not set to an instance of an object with a text box containing the following: at ...

I would bet that there are multiple ways to trigger the "Object reference not set..." message, but I don't know if there are ways to get a better traceback into the sw2urdf script. It might require ...