System.NullReference.Exeption in VB.NET

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VB

Hi,

I making the game 2048 but as an extra I wanted the users to let them change the height of the square if they wanted to. If the users click on the 'Verander spelbord' button a inputbox will show up where they can fill in the height of the square. Their input will then be shown at the textbox right under the button which is already filled in with 4. The height of the square will then change to the text in that textbox. Now when I try to run this I get an System.Nullreference.Exception but I have no idea why since the textbox is initially filled with a 4. Debugger says this: Object reference not set to an instance of an object.

And also I am a very newbie at Visual Basic so I know that this method of mine might not work. If someone could help me and tell me what method is best to implement a height changer to the game, I would be very thankful. PS: I'm Dutch so I'm sorry for the different language in the code and designer.

Public Class Form1


    Public square As Integer = (Convert.ToInt32(txtSquare.Text) - 1)

    Dim rooster(square, square) As Label
    Dim oldrooster(square, square) As Label

    Public clickEnabled As Boolean = False

    Public Sub btnOrde_Click(sender As Object, e As EventArgs) Handles btnSquare.Click
        Dim message, title As String
        Dim defaultValue As Integer
        Dim userInput As Object
        'Bericht op scherm.
        message = "Geef de hoogte van je spelbord in. (vb. 4 voor een 4x4 vierkant)"
        'Titel scherm.
        title = "Initialisatie spelbord"
        defaultValue = 4   'Standaardwaarde.

        ' Geef bericht, titel, en standaardwaarde.
        userInput = InputBox(message, title, defaultValue)
        'Als er wordt geannuleerd of als input kleiner is dan 2, dan blijft de hooghte de standaardwaarde (4). 
        If userInput = "" Then userInput = defaultValue
        If Not userInput > 1 Then userInput = defaultValue
        square = (Convert.ToInt32(userInput) - 1)
        txtSquare.Text = Convert.ToInt32(userInput)

    End Sub

[1]: http://i.stack.imgur.com/iQ0rc.png
[2]: http://i.stack.imgur.com/ooq3c.png