Object reference not set to an instance of an object.

i used a datalist within datalist i placed a panel and in the panel i placed a placeholder then a table in it .
i placed the lables, textbox and checkboxes in the table from backcode that is in c# code
i tried to find the control(Table) in datalist and also in placeholder but it shows null what is the reason
it gives the exception as in the subject
the table name and reference names are same as i declared
Table tbl = (Table)pnlMentalStatusExam.FindControl("tbl");
Table tbl = (Table)phMentalStatusExam.FindControl("tbl");
tbl is same as i declared in the table creation

pls solve my problem
Table tbl = (Table)pnlMentalStatusExam.FindControl("tbl");
Table tbl = (Table)phMentalStatusExam.FindControl("tbl");
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