The method getPropertyValue is invoked on null in this case, causing an error obviously. This happens if css() is used for an element, of which some parent element has its display property declared to "none".

The following patch fixes the nullpointer at least and returns an empty string instead (the retrieval of the CSS property in question is not fixed):

jQuery.swap(elem, { display: 'block' }, function() {
    var computedStyle = document.defaultView.getComputedStyle(this, '');
    ret = computedStyle && computedStyle.getPropertyValue(prop) || '';
});

As is now:

jQuery.swap(elem, { display: 'block' }, function() {
    ret = document.defaultView.getComputedStyle(this,null).getPropertyValue(prop);
});

Additional Information:

http://snook.ca/archives/javascript/safari2_display-none_getcomputedstyle/