Selectize throws error in Firefox if in an iframe with display:none

[wch]

When selectize is used in an iframe with display:none, it throws an error. It's due to this long-standing bug in Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=548397

The JS console says:

TypeError: window.getComputedStyle(...) is null selectize.min.js:2

The error is due to this line:
https://github.com/brianreavis/selectize.js/blob/6a071a2809802176ebc01703b98109784ec5b931/src/selectize.js#L7

In particular:

window.getComputedStyle(input, null).getPropertyValue('direction')

In the hidden iframe situation, window.getComputedStyle(input, null) returns null, but another function call is chained after it.

We've encountered this error on a different project, and our workaround is here:
rstudio/shiny#550