Cast from type DBNull to type String is not valid

[The Cheat]

I have an application where I am filling text boxes with data from a table. Every now and then, not all the fields in the table are going to contain data. For instance some records will have notes and others won't. Right now if I try and do this (below) I get Cast from type DBNull to type String is not valid

Try
                cnn.Open()
                dr = cmd.ExecuteReader
                While dr.Read()
                    tbRoNumb.Text = dr("RoNumber")
                    tbLastName.Text = dr("LastName")
                    tbFirstName.Text = dr("FirstName")
                    tbVehicle.Text = dr("Vehicle")
                    tbInsurer.Text = dr("Insurer")
                    tbColorCode.Text = dr("ColorCode")
                    tbAmount.Text = dr("Amount")
                    tbDateEntered.Text = dr("DateEntered")
                    tbDeliverDate.Text = dr("DeliveryDate")
                    tbDeliverTime.Text = dr("DeliveryTime")
                    tbPartsDate.Text = dr("PartsDate")
                    tbF.Text = dr("Frame")
                    tbM.Text = dr("Mech")
                    tbA.Text = dr("Align")
                    tbB.Text = dr("Body")
                    tbP.Text = dr("Paint")
                    tbReleaseDay.Text = dr("ReleaseDate")
                    tbReleaseTime.Text = dr("ReleaseTime")
                    CB1.Checked = dr("CB1")
                    CB2.Checked = dr("CB2")
                    CB3.Checked = dr("CB3")
                    CB4.Checked = dr("CB4")
                    CB5.Checked = dr("CB5")
                    tbMemo.Text = dr("Notes")
                End While
                cnn.Close()

How do I do this and still allow for nulls in one or more fields?