GUI Drag/Drop Segmentation Fault

[Griever]

I am having problems with a GUI I'm working on. The idea is to have a tree list of signals and be able to drag them onto the plot. Eventually having a long list of signals and multiple plots etc.. However the code segmentation faults after a seemingly random number of drag & drops (sometimes just one). I've stripped the code to the bare bones so it plots the same curve each time and there is only one "signal" to choose from; in this case just x^2.

Below I've posted the code with the packages it requires.

#===================================================================#
# Import Packages #
#=================#

import wx
import random
import matplotlib
matplotlib.use('WXAgg')
from matplotlib.backends.backend_wxagg import FigureCanvasWxAgg
from matplotlib.figure import Figure

#====================================================================#

class MainFrame(wx.Frame):
    ''' Create the mainframe on which all of the other panels are placed.
    '''
    def __init__(self):
        wx.Frame.__init__(self, parent=None, title="GUI", size=(998,800))
        self.SetBackgroundColour('#CCCCCC')
        self.GUIBox = wx.BoxSizer(wx.HORIZONTAL)        
        self.P = PlotWindow(self)
        self.DD = DragDrop(self)
        self.GUIBox.Add(self.DD, 0, wx.LEFT | wx.ALIGN_TOP)
        self.GUIBox.Add(self.P, 0, wx.LEFT | wx.ALIGN_TOP)
        self.SetSizer(self.GUIBox)      
        return

class PlotWindow(wx.Panel):
    def __init__(self, parent):
        wx.Window.__init__(self, parent)
        self.Figure = Figure()
        self.Figure.set_size_inches(8.56, 9.115)
        self.C = FigureCanvasWxAgg(self, -1, self.Figure)
        self.SP = self.Figure.add_subplot(111)
        self.a = [0,1,2,3,4,5]
        self.b = [0,1,4,9,16,25]
        self.signals = [self.b]

    def rePlot(self):
        self.SP.clear()
        self.SP.plot(self.a,self.signals[0])
        self.C.draw()

class MyTextDropTarget(wx.TextDropTarget):
    def __init__(self, objt):
        wx.TextDropTarget.__init__(self)
        self.Objt = objt

    def OnDropText(self, x, y, data):
        self.Objt.rePlot()

class DragDrop(wx.Panel):
    def __init__(self, parent):
        wx.Panel.__init__(self, parent, style=wx.BORDER_RAISED)
        self.SetBackgroundColour('#CCCCCC')
        self.tree = wx.TreeCtrl(self, -1, size=(270,700))
 
        # Add root
        root = self.tree.AddRoot("Signals")
        self.tree.AppendItem(root, "Square")        
        
        dt = MyTextDropTarget(self.GetParent().P)
        self.GetParent().P.SetDropTarget(dt)
        self.Bind(wx.EVT_TREE_BEGIN_DRAG, self.OnDragInit)
        
        self.VBox = wx.BoxSizer(wx.VERTICAL)
        self.VBox.Add(self.tree, 0)
        self.SetSizer(self.VBox)
        
    def OnDragInit(self, event):
        text = self.tree.GetItemText(event.GetItem())
        tdo = wx.TextDataObject(text)
        tds = wx.DropSource(self.tree)
        tds.SetData(tdo)
        tds.DoDragDrop(True)

#====================================================================#
# Create App #
#============#

class App(wx.App):
    def OnInit(self):
        self.dis = MainFrame()
        self.dis.Show()
        return True

app = App()
app.MainLoop()

#====================================================================#

I've tried to take out as much unnecessary code as possible; any help would be much appreciated!

Cheers!