3D FlipBook - Can't load PDF, pdf.min.js error

Suddenly it does not load the PDF and returns a "document.getElementById(...) is null" error.

Console show this error: details: "TypeError: document.getElementById(...) is null" message: "document.getElementById(...) is null" name: "UnknownErrorException" stack: "BaseExceptionClosure@https://www.fizzicseducation.com.au/wp-content/plugins/interactive-3d-flipbook-powered-physics-engine/assets/js/pdf.min.js?ver=2.5.207:1:[......]

Any suggestions on how to fix the issue will be greatly appreciated.