title: Python reference with Tkinter label change text
body: <p>I am trying to to change my Tkinter <code>Label</code> text from another function. Somehow I cannot reference the <code>Label</code> and therefore not changing the Label, only add a new on top of the old.</p>
  
  <p><code>self.errorLabel['text'] = 'second'</code> works inside the init-function of Window, but I need to be able to do it from another function, therefore a reference fault somehow.</p>
  
  <p>This is my code:</p>
  
  <pre><code>import sys
  from tkinter import *
  from tkinter import ttk
  
  class Window(Frame):
  
      def __init__(self, master = None):
          Frame.__init__(self, master)
          self.master = master
          self.errorLabel = Label(self, text="some text").grid(row=0)
          self.init_window()
  
      def init_window(self):
          self.master.title("Example")
          self.pack(fill=BOTH, expand=1)
  
          goButton = Button(self, text="Go!", command=self.client_go).grid(row=1, column=1)
          quitButton = Button(self, text="Close", command=self.client_exit).grid(row=1, column=0)
  
      def client_exit(self):
          exit()
  
      def client_go(self):
          self.errorLabel['text'] = 'second' # TypeError: 'NoneType' object does not support item assignment
          self.errorLabel = Label(self, text="second").grid(row=0)
          return
  
  root = Tk()
  root.geometry("800x500")
  app = Window(root)
  root.mainloop()
  </code></pre>