NL 12.10.0 onwards no longer supports LVL (or INI, or LEV) files, so the editor should not try to open them (as its ability to do so works on the basis of "get NL to convert it to an NXLV"), or indeed, should not offer the option to do so.
To be clear: This is not proposing removing any functionality. The editor never had the ability to open these files; it had the ability to ask NL to convert them to NXLV (and then open the resulting NXLV). NL no longer supports these formats, so this is essentially "the editor needs to stop asking it to try, and stop suggesting to the user that it can do so".