This may be a dup? I'm seeing this on pages with iframes. Presumably we're getting some web progress event from them, trying to get the associated browser and then use it. We could probably just see we have no browser and back off. We don't want to send progress events from iframes to java anyway... do we? Maybe on sites like gmail we want to show a spinner while the iframe loads?
Created attachment 573378 [details] Testpage This is just a page with an iframe to reproduce the error on.
This was happening in the DOMContentLoaded event handler. We looked for the <browser> associated with the event.target (HTMLDocument) but didn't find one. Then we try to set focus to the null browser.
Created attachment 573833 [details] [diff] [review] patch No errors using the test page anymore. I stopped trying to focus the browser too. We don't need it. The keyboard comes up fine on Google and Bugzilla without forcing focus.
Created attachment 573896 [details] [diff] [review] patch 2 adds more checks