window.getDefaultComputedStyle may return null

In Iceweasel 24.2.0 it happens here when I call layout from jQuery layout plugin in an iframe from an invisible tab (from jQuery UI tabs).

I know it's not in the API specification but believe me: it may return null. Here's the message I get in the console of Iceweasel 24.2.0:

"TypeError: window.getDefaultComputedStyle(...) is null"