object reference not set to an instance of an object error

the datatable in this code give me error of
object reference not set to an instance of an object



 Dim dt As DataTable = New DataTable
        dt = DataGridView1.DataSource
        For a = 0 To DataGridView1.Rows.Count - 1 Step 1
            If a = DataGridView1.Rows.Count - 1 Then
                GoTo v
            End If
            'myobjects._emp_num = DataGridView1.Rows(a).Cells(0).Value
            myobjects._emp_num = dt.Rows(a)("emp_id")
            myobjects._emp_name = dt.Rows(a)("emp_name")
            myobjects._emp_situation = dt.Rows(a)(3)
            myobjects._day_date = dt.Rows(a)(2)
            com.add_al7door(myobjects)

        Next
v: