santosh86 says:
AjaxControlToolkit.TabContainer ctrl = FindControl("GeneratePackage") as AjaxControlToolkit.TabContainer;
replace above code with below
AjaxControlToolkit.TabContainer chklstFSOptions = this.Page.FindControl("YourTabContainerIDGoesHere") as AjaxControlToolkit.TabContainer;
Just replace the Id as per your TabContainer Id.