fix:cannot call method “indesOf” of “undefined”

when I use this project to parse a xlsx file,it throw an error which says "cannot call method “indesOf” of “undefined”",and force in 107 clo--f = numFmts[id];I run my project ，and console.log the value of "f" and
someTimes f will be an undefined and it cause  the xlsx file cannot be parse.