There's a race condition in showObjectPopover() -- sometimes, popover may be hidden while a popover expression is being evaluated in the inspected page. In this case, _highlightElement becomes undefined and this._highlightElement.addStyleClass() call in showObjectPopover() results in an exception.
Created attachment 112323 [details] patch
A variation of the patch landed manually as r98333: http://trac.webkit.org/changeset/98333