Invalidcasrexception was unhandle. Conversion from type 'dbnull' to type 'string' is not valid

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VB

What is wrong with the coding, when i try to run, it say invalidcastexception : Conversion from type 'DBNull' to type 'String' is not valid. what i have to do

What I have tried:

Public Class frm_invoice_a155960

Dim i As String

Private Sub frm_invoice_a155960_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
refresh_grid()
refresh_grid2()

End Sub

Private Sub refresh_grid()

Dim last_id As String = run_sql_query(" SELECT MAX(FLD_ORDER_ID) AS LAST_ID FROM TBL_ORDER_LIST_A155960").Rows(0).Item("LAST_ID")
Dim last_customer As String = run_sql_query(" SELECT MAX(FLD_CUSTOMER_ID) AS LAST_NAME FROM TBL_ORDER_A155960").Rows(0).Item("LAST_NAME")
Dim last_date As String = run_sql_query(" SELECT MIN(FLD_ORDER_DATE) AS LAST_DATE FROM TBL_ORDER_A155960").Rows(0).Item("LAST_DATE")
lbl_order.Text = "" & last_id
iname.Text = "" & last_customer
lbl_date.Text = "" & last_date

Dim mysql As String = " SELECT * FROM TBL_ORDER_LIST_A155960 WHERE FLD_ORDER_ID LIKE ""%" & lbl_order.Text & "%"""
Dim mydatatable As New DataTable
Dim myreader As New OleDb.OleDbDataAdapter(mysql, myconnection)
myreader.Fill(mydatatable)

grd_invoice.DataSource = mydatatable

grd_invoice.Columns(0).HeaderText = "ID PRODUCT"
grd_invoice.Columns(1).HeaderText = "ID ORDER"
grd_invoice.Columns(2).HeaderText = "QUANTITY"
grd_invoice.Columns(3).HeaderText = "PRICE (RM)"
grd_invoice.Columns(4).HeaderText = "SUBTOTAL (RM)"


End Sub

Private Sub refresh_grid2()

Dim total As String = 0
For I As Integer = 0 To grd_invoice.RowCount - 1
total += grd_invoice.Rows(I).Cells(4).Value

Next

lbl_total.Text = "RM " & total

End Sub
End Class