Fix vertical scroll breaking after window resize

In rare cases when using gridviewScroll in full screen mode, and resizing the gridview on window resize you can cause the vertical scroll to break.

The use case I ran into is, if the window is resized to be smaller, causing the horizontal scroll bar to appear freezeitemcontent is null.  
Calling freezeitemcontent.scrollTop(delta); throws an exception since freezeitemcontent is null, and vertical scrolling stops.

Adding a null check solves this issue.