opening ui dialog does not work for the second time

At first I tried to reopen http://dev.jqueryui.com/ticket/5185 - but I don't have permission to do so...

(related to changeset http://dev.jqueryui.com/changeset/3823)

as we updated from jquery ui 1.7.2 to 1.8rc3 we noticed that the dialog widget does not work for the second time opened(1, 3-... times work).

tracking the error down we found out that it happens in the line:

625: var $el = (this.oldInstances.length ? this.oldInstances.splice(0, 1)[0] : $('<div></div>').addClass('ui-widget-overlay'))

the problem is that the

oldInstances

array contains a valid element (used for the first opening of the dialog) and a second element that has the value __undefined__ - and this is the problem as the above expression tries to call .appendTo(document.body) on the array element