ArrayList in VB.NET Class

I am creating my first ever custom class.

Code currently us...

Public Class AQL_SMS
    Public Username As String
    Public Password As String
    Public Destination As ArrayList
    Public Message As String
    Public Originator As String
    Public Max_Concat As String
    Public Sendtime As Date
    Public Unix_Sendtime As Integer
    Public ReplaceSMS As Boolean
    Public Flash As Boolean
    Public Dlr_URL As String

    Public Sub New(ByVal _username As String, ByVal _password As String)
        Username = _username
        Password = _password
    End Sub


    Public ReadOnly Property Destinations As String
        Get
            Dim tmpjoin As String = ""
            Dim str As String
            For Each str In Destination
                tmpjoin = String.Concat(tmpjoin, str.ToString & ",")
            Next
            tmpjoin = tmpjoin.Substring(0, tmpjoin.Length - 1)
            Return tmpjoin
        End Get
    End Property
End Class

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This is a class based upon http://www.aql.com/sms/integrated/sms-api/

I have set Destination as an ArrayList as it should be able to contain multiple destination numbers.

I have declared in my progect code as follows...

Public Class frmClassView
    Public SMS As New AQL_SMS("my_username", "my_password")

    Private Sub btbSMS_Click(sender As System.Object, e As System.EventArgs) Handles btbSMS.Click
        SMS.Destination.Add("01234567891")
        SMS.Destination.Add("01234567891")

        tmrAutoSMSPropertyCheck.Start()
    End Sub

    Private Sub tmrAutoSMSPropertyCheck_Tick(sender As System.Object, e As System.EventArgs) Handles tmrAutoSMSPropertyCheck.Tick
        lblUsername.Text = SMS.Username
        lblPassword.Text = SMS.Password
        lblDestination.Text = SMS.Destinations
        lblMessage.Text = SMS.Message
        lblOriginator.Text = SMS.Originator
        lblMax_Concat.Text = SMS.Max_Concat
        lblSendtime.Text = SMS.Sendtime
        lblUnix_Sendtime.Text = SMS.Unix_Sendtime
        lblReplaceSMS.Text = SMS.ReplaceSMS
        lblFlash.Text = SMS.Flash
        lblDlr_URL.Text = SMS.Dlr_URL
    End Sub
End Class

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However I get the error...

System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=WindowsApplication1
  StackTrace:
       at WindowsApplication1.frmClassView.btbSMS_Click(Object sender, EventArgs e) in C:\Users\terry\AppData\Local\Temporary Projects\WindowsApplication1\frmClassView.vb:line 5
       at System.Windows.Forms.Control.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ButtonBase.WndProc(Message& m)
       at System.Windows.Forms.Button.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
       at WindowsApplication1.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException:

at the

SMS.Destination.Add("01234567891")

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line.

Why, and how do I fix this to work as I expect it to?