Cannot read property ‘toString’ of undefined

Console report following error making the entire solution unusable:

Uncaught TypeError: Cannot read property ‘toString’ of undefined
at checkOfssets (app.js?ver=5.1.1:158)
at HTMLSpanElement.<anonymous> (app.js?ver=5.1.1:91)
at Function.each (jquery.js?ver=1.12.4:2)
at a.fn.init.each (jquery.js?ver=1.12.4:2)
at new Sliders (app.js?ver=5.1.1:49)
at HTMLDocument.<anonymous> (app.js?ver=5.1.1:363)
at i (jquery.js?ver=1.12.4:2)
at Object.fireWith [as resolveWith] (jquery.js?ver=1.12.4:2)
at Function.ready (jquery.js?ver=1.12.4:2)
at HTMLDocument.K (jquery.js?ver=1.12.4:2)

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