The "console.show()" message is a symptom, since it's called in the catch block of validateCompletedForm() when anything goes wrong. console.show() seems to be an Adobe Acrobat-specific thing, it does not appear in the scripting API.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0cb6b59e39bce4769dfda830cf435b4400086ffd

commit 0cb6b59e39bce4769dfda830cf435b4400086ffd
Author: Henrique Nakashima <hnakashima@chromium.org>
Date: Mon Aug 20 18:46:15 2018

Handle FFI_PageEvent from PDFium in PDF Viewer.

Add or remove pages from the viewer in response to these events.

Bug:  pdfium:401 , chromium:872903 ,chromium:867135
Change-Id: Iddc88c5a90370213619a0f3f2587f215d0dafbc8
Reviewed-on: https://chromium-review.googlesource.com/1169691
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584515}
[modify] https://crrev.com/0cb6b59e39bce4769dfda830cf435b4400086ffd/pdf/pdfium/pdfium_engine.cc
[modify] https://crrev.com/0cb6b59e39bce4769dfda830cf435b4400086ffd/pdf/pdfium/pdfium_engine.h
[modify] https://crrev.com/0cb6b59e39bce4769dfda830cf435b4400086ffd/pdf/pdfium/pdfium_form_filler.cc
[modify] https://crrev.com/0cb6b59e39bce4769dfda830cf435b4400086ffd/pdf/pdfium/pdfium_form_filler.h

When I bypass the isNull error by editing the PDF and working around it, this works. So this is not a case of we're seeing the tip of the iceberg, it's actually only blocked in one more issue.