FLS 5.04 in Windows can crash occasionally when closing the metrics window. It seems to be a cumulative problem. If you have multiple files open and you're opening and closing metrics windows often, your chances of crashing seems to increase. If you're getting this type of crash often, try clearing the text field before closing the window.
One thing y9u should never to is save then close the metrics window in quick succession. If you do, FontLab may crash before the save has been completed which will corrupt the file. Save . . . wait a few seconds, close the metrics window.