title: TclError: can't invoke "destroy" command: application has been destroyed
body: <p>I am a python beginner. Try to make a new button to close the window. I got the error message:                                      </p>
  
  <blockquote>
    <p>Exception in Tkinter callback
    Traceback (most recent call last):   File
    "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk/Tkinter.py",
    line 1536, in <strong>call</strong>
        return self.func(*args)   File "tk_cp_successful.py", line 138, in buttonPushed
        self.root.destroy()   File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk/Tkinter.py",
    line 1859, in destroy
        self.tk.call('destroy', self._w) TclError: can't invoke "destroy" command:  application has been destroyed</p>
  </blockquote>
  
  <pre><code>class LoginPage(tk.Frame):
     def __init__(self, parent, controller):
        self.controller = controller
        self.root = tk.Tk()
        global entry_1
        global entry_2
        tk.Frame.__init__(self, parent)
        label = tk.Label(self, text="Welcome to VISA Login Page",fg="blue")
        label.pack(pady=10,padx=10)
  
        label_1 = Label(self, text="Username")
        label_1.pack()
        label_2 = Label(self, text="Password")
        label_2.pack()
        entry_1 = Entry(self)
        entry_1.pack()
        entry_2 = Entry(self, show="*")
        entry_2.pack()
        label_1.grid(row=0, sticky=E)
        label_1.pack()
        label_2.grid(row=1, sticky=E)
        label_2.pack()
        entry_1.grid(row=0, column=1)
        entry_1.pack()
        entry_2.grid(row=1, column=1)
        entry_2.pack()
        checkbox = Checkbutton(self, text="Keep me logged in")
  
        checkbox.grid(columnspan=2)
        checkbox.pack()
        logbtn = Button(self, text="Login", command = self._login_btn_clickked)
        logbtn.grid(columnspan=2)
        logbtn.pack()
        myButton = Button(self, text="Exit",command = self.buttonPushed)
        myButton.pack()
  
    def buttonPushed(self):
        self.root.destroy()
  
    def _login_btn_clickked(self):
        #print("Clicked")
        username = entry_1.get()
        password = entry_2.get()
  
        #print(username, password)
  
        if username == "test" and password == "test":
            #box.showinfo("Login info", "Welcome Tester")
            button1 = ttk.Button(self, text="Please click, Welcome to login!!!",
                       command=lambda: self.controller.show_frame(StartPage))
            button1.pack()
        else:
            box.showerror("Login failed", "Incorrect username")
  </code></pre>