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()