get data from dynamically generated textboxes

am facing the problem in accesing the dynamically generated textboxes

i am showing some data in gridview when i click on edit in some record a new panel is set to visible which contain form to update data

there is a single-column in db which contain many different values so to update it i split the values generate the textboxes at runtime and show it in textboxes now i want to know that when i click update how should i retrive values again from these textboxes

textbox name: tbin

tbin.id = "time"+i.toString()

placeholder name = placeholder6

in update btn i am doing this : string valin = ((TextBox)PlaceHolder6.FindControl("timein" + i.ToString())).Text;

but it gives null refrence error