I've figured out the problem! Actually the DNN page has a form with one hidden input element named 'ScrollTop' that is manipulated by the function __dnn_bodyscroll() in dnncore.js; The first line inside the function is

var oF = document.forms[0]; // pointing to first form in the collection

Since it has only one form so there's no problem in scrolling.

When addthis social bookmark button is activated by hovering or clicking, it creates another form so now there are 2 forms in the collection and forms[0] is pointing to newly created form which doesn't have that hidden input element named 'ScrollTop' so we are getting 'ScrollTop is null or not an object' message.

The solution to this problem is to change __dnn_bodyscroll() function in dnncore.js in a way so that it should point to correct form in the dnn page who has this hidden input element named: 'ScrollTop'

In my case, the form has name and id = 'form' so I changed

var oF = document.form;

and it worked very well.