I found the problem, I had to insert this script in the Parent.Page
, instead of this
because it was trying to find the gridview in the popup, so it wasn't finding it.
Now works:
ScriptManager.RegisterStartupScript(this.Parent.Page, Parent.Page.GetType(), "Javascript", "aspxgridEditLog.PerformCallback(); ", true);
Thanks for the guesses, you helped me think about what was the problem.