I'm seeing this error happen in porting browser_switchTab_decodeuri.js to QuantumBar. I can't reproduce it manually yet.

JavaScript error: resource:///modules/UrlbarView.jsm, line 586: TypeError: item is undefined

https://searchfox.org/mozilla-central/rev/2a6f3dde00801374d3b2a704232de54a132af389/browser/components/urlbar/UrlbarView.jsm#586

From the looks of it, we're triggering UrlbarView._on_SelectedOneOffButtonChanged when the popup is hidden, and we're setting the selected engine to null.

My guess is that the rows may have gone away, but we've still got the queryContext.

The SelectedOneOffButtonChanged can fire when the popup is closed, so we should ignore it then. browser_switchTab_decodeuri.js hits this, as Mark says in the bug. The stack is:

_on_SelectedOneOffButtonChanged@resource:///modules/UrlbarView.jsm:569:131
handleEvent@resource:///modules/UrlbarView.jsm:298:23
dispatchEvent@chrome://browser/content/search/search-one-offs.js:147:20
set selectedButton@chrome://browser/content/search/search-one-offs.js:306:10
_on_popuphidden/<@chrome://browser/content/search/search-one-offs.js:1281:7

Alternatively we could modify the selectedButton setter in the one offs not to fire the event when the popup is closed. However, theoretically someone could be interested in listening for the selected button change even then, so I think it's better to modify the listener here.

https://hg.mozilla.org/mozilla-central/rev/5bda26248e68