Unregistered Refresher (575) that was already unregistered

I’m trying to use the Refresher extension outlined in
https://vaadin.com/wiki/-/wiki/Main/Creating+an+UI+extension

When connected with Google Chrome, I periodically see a spew of these warnings:

WARN http-bio-8080-exec-1: Unregistered Refresher (575) that was already unregistered. [com.vaadin.ui.ConnectorTracker]

The browser ultimately gives a “Communication problem. Take note of any unsaved data, and click here to continue. Server error. Error code: 500” error, although no indication of this is on the sever side.

This does not happen with Firefox.

Running on Vaadin 7.1.6 and Tomcat 7.0.