Torbutton: Unexpected error on new identity: TypeError: b.webProgress is undefined

Anybody yet observes this alert? TBB-3.6 Everything worked fine before, and then suddenly alert message box. Last automatically updated add-on was Noscript 2.6.8.23

Debugging showing that exception generated by

browser.addTab("about:blank");

from torbutton_close_on_toggle()

Can't reliably reproduce this bug, everything works and then for the same sequence of actions this fail appears.