Thanks for the suggestions, but the deletion of those files
did not help to fix the problem. I tried them in succession, and
then tried to delete all three files at once and open the project.
The loss of those files managed to delete my user-defined
variables, but I was working from a backup copy, so no worries
there... : )
I did notice that if I used Alt + Enter with the "none" node
selected, I could look at the properties for the "none" node.
RoboHelp told me, through the "Used In" tab, that "none" was used
by a specific file within the same project directory.
I've looked at that file, and there is nothing at all within
the file to suggest a reference to this empty "none" node. I looked
at all hyperlinks in the file to make sure they all pointed to an
existing topic... check.
I took a look at topic references for the file that appeared
in the "Used In" tab, and there is nothing out of the ordinary
there.
I'm still looking for a fix for this very weird behavior.