I'm not clear on what you are asking.
to insert a null value
insert into table
(column1, column2, column3)
values
(23, null,'Hello world')
Your textbox.text = implies you are using some programming language which has retrieved the data and are trying to display it, but have an issue if the data returned is null.
without knowing what programming language, any help needs to be generic - but you need to do something like
If row("column2") is DbNull.Value Then
TextBox1.Text = String.empty
Else
TextBox1.Text - row("Column2")
End If