Prevent a "TypeError: pdfViewer is null" when the viewer loads (PR 5413 followup)

Currently if you manage to e.g. open the console (with <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>K</kbd>) before the viewer is initialized, the following will be printed in the console: `TypeError: pdfViewer is null`.
This doesn't cause any actual errors, but the printing of this kind of message nevertheless seems like something we should avoid.

Followup to PR 5413.