To avoid the crash you should create the matrix before you open the file dialog...
window -t matrix; fdlog.optionDlg$=IMPASCII; fdlog.usegroup(ASCII); fdlog.open(); %B=fdlog.path$; open -w B;
...Apparently ASCII options are not saved with matrix templates. (Never noticed that before!) One possibility is to import to a wks (for which you can save ASCII options) and then convert to matrix. See my reply to your other topic for details.