"Object reference not set to an instance of an object" error occurs when try to get/set null value.
In your code , this situation might be occurs many places
1.
ComboBox1.SelectedValue.ToString
ComboBox1.SelectedValue does not fetch any value.
2.
da.Fill(dt)
when you try to fill data adapter
..............
so, better to debug your code to find actual place of source of error
or use try, catch block with your code
1 more thing-
always use
Convert.Tostring()
in the place of
yourvalue.ToString()