Hi,
I as well have come across the same predicament.
A couple of things though:
When working on my site via MAMP – localhost, the Page Lists Plus widget worked tremendously. When I brought the site “to life”, unfortunately I started to get the same errors.
Also, I am no WP genius by any means, and when it comes to the coding of WP sites, this “Notice” really doesn’t mean anything to me.
In search of another widget, I came across code to paste in “here” or there, but like I said… I’m no WP genius. Yet.
Was yours working prior to the install of the plugin?
I really enjoyed it when it was working, as the unlink JS feature was golden, and updating the styles without this feature tended to be quite the struggle.
Hopefully we can remedy this soon.
Regards