We can't tell - and we can't look, because we don't have access to your query, or to your data.
There are four basic possibilities:
1) Your query does not return a table called
Tbl_JobseekerPersonalDetails
2) Your query returns the table, but there are no rows in the table
3) Your query returns the table, and it has rows, but there is no column called
JobseekerID
4) Your query returns the table, and it has rows, and there is a column, but
TextBoxID
is null.
Which of these it is is up to you: use the debugger, look at your code when the problem occurs and find out which of these is the case.
If you can't work the debugger, then you need to learn, but you can help find out by splitting the line:
var dt = ds.Tables["Tbl_JobseekerPersonalDetails"];
var row = dt.Rows[0];
var col = row["JobseekerID"];
TextBoxID.Text = col.ToString();
Which line it fails on will tell you which problem it is: you can then look back in your code to find out
why is it null.