关于PDFJS不能使用的问题

Question

我使用的是pdfjs-2.0.943；报错如下：Uncaught (in promise) TypeError: Cannot read property 'firstElementChild' of null
at PDFViewer.BaseViewer (base_viewer.js:136)
at new PDFViewer (viewer.js:8611)
at Object._callee5$ (app.js:318)
at tryCatch (runtime.js:62)
at Generator.invoke [as _invoke] (runtime.js:296)
at Generator.prototype. [as next] (runtime.js:114)
at step (app.js:1)
at app.js:1
at new Promise ()
at Object. (app.js:1)