Can't access a null value ...error

Hi,

I am using datagrid (which is in my library) it is not on my stage. I want to change its alpha property to .5 using greensock as3. I am getting error with the following code. I don’t know how to modify the following code when it is in library.

TweenMax.to(data_grid, 1, {alpha:.5, delay:1});

Thanks.