error occured when clicking a programatically added button [duplicate] - vb.net

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What is a NullReferenceException, and how do I fix it?
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I am trying to programatically generate buttons and add event handlers to them and i've succeeded in creating the buttons but when I click on one of the I get the following message object reference not set to an instance of an object .
Here is the code that creates the buttons:
Dim i As Integer = 1
For Each dtab As DataTable In ds.Tables
For Each drow As DataRow In dtab.Rows
If (CInt(drow(3)) < CInt(drow(4))) Then
Dim l As New Label()
With l
.Location = New System.Drawing.Point(30, 40 * i)
.Text = "product number " & drow(0) & " has reached the minimal quantity"
.Width = 300
Panel3.Controls.Add(l)
End With
Dim b As New Button()
With b
.Location = New System.Drawing.Point(350, 40 * i)
.Enabled = True
.Width = 100
.Text = "fill stock"
Panel3.Controls.Add(b)
AddHandler b.Click, AddressOf Me.Button_Click
End With
i = i + 1
End If
Next
Next
the code that handels the button click is the following:
Private Sub Button_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Dim co As New SqlConnection("server=JAMMINGJACK-PC\SQLEXPRESS ;initial catalog=td3;integrated security=true;")
Try
Dim cmd As New SqlCommand
co.Open()
cmd.Connection = co
cmd.CommandText = "Update article set qteStck=(qteStck+(seuilmin*3)) where numart=" & drow(0) '
cmd.CommandType = CommandType.Text
cmd.ExecuteNonQuery()
co.Close()
Catch ex As Exception
MsgBox(ex.Message, MsgBoxStyle.Critical, Me.Text)
End Try
End Sub

Answer 1

There are two big problems on this line in your event handler:
cmd.CommandText = "Update article set qteStck=(qteStck+(seuilmin*3)) where numart=" & drow(0) '
The first problem is the code is crazy vulnerable to sql injection attacks. It's a side-issue to the question, but this is IMPORTANT, and you should take a few minutes to Google parameterized queries and how to use them.
The second problem is with the drow(0) expression. That variable has no meaning in this method. It was scoped to the For Each loop in the code that created your buttons, but it has no meaning at all here. That this compiles at all means you either tried to re-declare it at the class level (which won't help; that will be a different drow variable) or turned Option Strict off, which is very poor practice.
There are several ways you can address this, such as encoding the drow(0) value in theTag property or inferring it from the Name. My preferred approach here would be to create a custom control that groups your label and button together.