wxPython - Delete() method in wx.TreeCtrl

import wx

class TreePanel(wx.Panel):

    def __init__(self, parent):
        wx.Panel.__init__(self, parent)

        # initialize Tree Control
        self.tree = wx.TreeCtrl(self, wx.ID_ANY, wx.DefaultPosition, (100, 150),
                                                 wx.TR_HAS_BUTTONS)

        # create Tree Control using Create() method
        self.tree.Create
        # Add root to Tree Control
        self.root = self.tree.AddRoot('Root')

        # Add item to root
        self.itm = self.tree.AppendItem(self.root, 'Item')

        # Add item to 'itm'
        self.si1 = self.tree.AppendItem(self.itm, "Sub Item")

        # Add another item
        self.si2 = self.tree.AppendItem(self.itm, "Another Sub Item")

        # Expand whole tree
        self.tree.ExpandAll()

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(self.tree, 0, wx.EXPAND)
        self.SetSizer(sizer)

        # Add button in frame
        self.btn = wx.Button(self, 1, "Delete", (10, 170))
        
        # Bind event function with button
        self.btn.Bind(wx.EVT_BUTTON, self.onclick)

    def onclick(self, e):
        # Delete si2 from the tree
        self.tree.Delete(self.si2)


class MainFrame(wx.Frame):

    def __init__(self):
        wx.Frame.__init__(self, parent = None, title ='TreeCtrl Demo')
        panel = TreePanel(self)
        self.Show()


if __name__ == '__main__':
    app = wx.App(redirect = False)
    frame = MainFrame()
    app.MainLoop()