Problem in using nitobi.callout

[Mangesh]

Hi,
I am trying to use nitobi.callout for validations of a grid on my
page. I want to show callout on a defaulter grid cell which will
ellaborate the error. I think nitobi.callout.Hint can satisfy my
requirement very well but facing some issues in using it. As shown in
example on http://nitobi.com/products/callout/hints/ I attached
callout to my element through my JS validation function. Now, when I
do mouseover on my grid cell, nitobi.callout.Callout.getStyleSheetUrl
function from nitobi.callout.js file gets called. In this function,
'nitobi.html.Css.getRule(nitobi.callout.lastStyle); returning null and
assigning null value to var _35. Hence it is throwing error on the
very next line where _35.parentStyleSheet is called.
Am I missing anything? Do I need to include anything more? What is
nitobi.callout.lastStyle and when, where and how it is needed to be
specified?

Thanks and Regards,
Mangesh