[R-sig-Debian] Warning messages in terminal after running help.start() and accessing the package index

[Ivailo]

Dear list-members,

I have switched (back) to a terminal-based workflow for a while now
and have noticed that after starting the HTML-based help
[help.start()], then visiting the package-list (at
http://127.0.0.1:<random_port>/doc/html/packages.html), and after the
accessed page has been "generated" (as indicated by the message
"Making 'packages.html' ... done"), as soon as the first command is
entered at the command prompt the following warning appears:

Warning messages:
1: In is.na(x) : is.na() applied to non-(list or vector) of type 'NULL'
2: In is.na(x) : is.na() applied to non-(list or vector) of type 'NULL'

This happens on both R-3.4.4 and R-3.5.1 running in XFCE-Terminal and Sakura.

Is there a way to suppress/fix the reason for these warning messages?

Cheers,
Ivailo