How do i...convertion type from dbnull to integer is not valid pls help me

Dim con As New SqlConnection("Data Source=NAAZNEEN-PC;Initial Catalog=testmob;Integrated Security=false;user id=sa;password=q1w2e3r4/;")

    Public conn As New ADODB.Connection
    Public rs As New ADODB.Recordset
    Public sql As String
    Public qry As String
    Dim image As String
    Dim cmd As SqlCommand
    Dim dr As SqlDataReader
    Dim da As SqlDataAdapter
    Dim ds As DataSet
    Public Function opendb() As Object

        If conn.State = 1 Then conn.Close()
        conn.Open("Provider=SQLOLEDB.1;Persist Security Info=False;User ID=sa;password=q1w2e3r4/;Initial Catalog=testmob;Data Source=NAAZNEEN-PC")
        Return 0
    End Function
    Sub loadcat()
        sql = "select * from tbl_category"
        If rs.State = 1 Then rs.Close()
        rs.Open(sql, conn)
        drpcategory.Items.Clear()
        Do While Not rs.EOF
            drpcategory.Items.Add(rs(1).Value)
            rs.MoveNext()
        Loop
    End Sub
    Sub loadid()
        Dim i As Integer
        sql = "select max(p_id) from tbl_product"
        If rs.State = 1 Then rs.Close()
        rs.Open(sql, conn)
        If rs.EOF = False Then
            i = rs(0).Value
            i = i + 1
            txtid.Text = i

        Else
            i = 1
            txtid.Text = i
        End If

    End Sub
    Sub clear()
        drpcategory.SelectedIndex = 0
        txtmodel.Text = ""
        txtcolor.Text = ""
        txtdesc.Text = ""
        txtamount.Text = ""
        txtquan.Text = ""
        txtid.Text = ""
    End Sub
    Sub loadgrid()


        qry = "select p_id as PRODUCT_ID,p_cat as CATEGORY,p_model as MODEL,p_color AS COLOUR,p_desc AS DESCRIPTION,p_amount as AMOUNT,p_quantity as QUANTITY,p_image as IMAGE from tbl_product"
        da = New SqlDataAdapter(qry, con)
        ds = New DataSet
        da.Fill(ds, "tbl_product")
        GridView1.DataSource = ds
        GridView1.DataMember = ds.Tables(0).ToString
        GridView1.DataBind()

    End Sub