System.dbnull.value null? APP not start if column null

Question

If is Column 1, Column 3 or Column 4 null app not start...

How tu insert in this code for system db null or recomandition???

 

openCon.Open();
using (SqlDataAdapter querySaveStaff = new SqlDataAdapter(saveStaff, cs))
{
     DataTable dt = new DataTable();
     querySaveStaff.Fill(dt);
     prijava_radnikaDataGridView.DataSource = dt;
     prijava_radnikaDataGridView.SelectionMode = DataGridViewSelectionMode.FullRowSelect;
     prijava_radnikaDataGridView.Columns[0].Visible = false;  //id
     prijava_radnikaDataGridView.Columns[1].FillWeight = 80;  //redni broj
     prijava_radnikaDataGridView.Columns[2].FillWeight = 250;  //radnik
     prijava_radnikaDataGridView.Columns[3].FillWeight = 180;  //vrijeme prijave
     prijava_radnikaDataGridView.Columns[4].FillWeight = 180;  //vrijeme odjave