Russell,

I'm not an expert on TS, but I think your CATCH code is dangerous in that it does not handle the case where a user cancels the GETPRINTER() dialog (returning '') or where your 2nd SET PRINTER command throws its own error.

I would try wrapping your CATCH code in its own nested TRY/CATCH. My changes in lowercase. Untested.

TRY
  SET PRINTER TO NAME (PrinterName)
CATCH
   try
     local lcPrinter
     lcPrinter = GETPRINTER()
     if not empty( lcPrinter )
       set printer to (lcPrinter)
     endif
   catch
     messagebox( "Problem with printer selection ...", ... )
   endtry
ENDTRY

Malcolm