FontView uses default font widths if HVAR table does not have a width mapping table. ttgxvar.c::ft_var_load_hvar() seems to survive loading the table, and reasonably sets face->blend->hvar_table->widthMap.mapCount to 0.

However, tt_hadvance_adjust() at line 807 then does the check:
if ( gindex >= face->blend->hvar_table->widthMap.mapCount )
{
FT_TRACE2(( "gindex %d out of range\n", gindex ));
error = FT_THROW( Invalid_Argument );
goto Exit;
}

The width mapping table is optional in the HVAR table; and the Adobe Type group now has two test fonts which show this issue.

By the way I am not familiar with the gyp build system. What is the simplest way to get a debug build, non-optimized and with all symbols? Debugging the regular build with lldb was a bit challenging. Do I edit 'common.gypi' or edit build.py so that args.release is set to '', or something else?