Hi,
It sounds like its probably some kind of timing issue, possibly caused by how/when you are loading WebViewer.
I would suggest adding some logs and breakpoints throughout your code to check for timing issues. Make sure that WebViewer is finished loading before trying to use this.wvInstance.
Also, if you are using some kind of Tab or Navigation library, it may be setting elements to display:none without you directly knowing it. You could make sure this is not the case either.
I do not believe this issue is being caused directly by PDF.js Express.
Let me know your findings!
Thanks,
Logan