Thanks for the bug report Andy, I'll see about getting the crash fixed up in v5.

One thing to note though is that this is not the kind of data that the sub-d import tool is expecting to work on. It's not a sub-d control cage, it's a whole bunch of triangles and isn't configured for having sub-d smoothing applied to it.

A Sub-d model should be formed of all quads and have a sparser cage structure to it, see here for some examples:
https://vimeo.com/2450612

MoI and just CAD programs in general are not designed to work with this kind of dense faceted triangulated data. You need to use a polygon mesh editing program to work with that form of data, MoI isn't the right tool for it.

- Michael