Public f1 As Form1 ' ist gesetzt.
 
Private Sub Anhängen()
  Dim pn As TreeNode
  pn = f1.TreeView1.Nodes(1) ' Fehler Additional information: Object reference 
  ' not set to an instance of an object.
  Dim cn As New TreeNode
  cn.Text = "Neu"
  cn.Tag = "Node1\Neu"
  pn.Nodes.Add(cn)
End Sub