How to uncheck a checkbox after the close_event of figure PYTHON

This is my first question, so please let me know if I make any faux pas. I'd like to have a check-box that creates and distroys a plot. If that plot is closed using the window's "x", then I'd like the check-box to be unchecked. I've got it all working except the unchecking upon figure closing. It looks like my close function is getting called, but it crashes when I try to manipulate the check-box.

from tkFont import *
from Tkinter import *
import matplotlib.pyplot as plt
import numpy as np

class Application(Frame):

    def velocity_cb(self):
        if self.velocity_var.get()==1:
            print "selected Velocity checkbox"
            x = np.arange(100.)
            self.figureHandles[1] = plt.figure("Velocity")
            # set the close event to run my function
            self.figureHandles[1].canvas.mpl_connect('close_event', self.handle_close)
            plt.plot(x,x)
            plt.show()
        else:
            print "deselected"
            plt.close(self.figureHandles[1])

    def handle_close(self,event):
        #print self.ground_var.get()
        print "X'd velocity window"
        #print self.figureHandles[1]

        #plt.close(self.figureHandles[1])
        self.velocity_var.set(0)   

    def createWidgets(self):
        self.figureHandles = [0]*13
        self.velocity_var = IntVar()
        self.velocity = Checkbutton(self, text="Velocity", variable=self.velocity_var, command=self.velocity_cb)
        self.velocity.grid(row=6,column=0,sticky='W')
        self.velocity.var = self.velocity_var

    def __init__(self,master=None):
        Frame.__init__(self,master)
        self.pack()
        self.createWidgets()            

if __name__=='__main__':
    root = Tk()
    root.wm_title("FireFLY6 Plotting Tool")    
    app = Application(master = root)
    app.mainloop()