The SpellChecker Class currently has no publicly visible way of accessing the closed field. It also calls close in the finaliser killing the process it is in upon GC as this can throw an exceptin. I propose two changes:
Change the already existing method "IsClosed()" to public:
public bool IsClosed()
and add a check on this in the finaliser:
Ideally this class should implement IDisposable but I think this would be a bigger job than this two line change.