title: AttributeError: 'NoneType' object has no attribute '_root'
body: <p>I am completely new to Python 3, just following simple exercises from YouTube.</p>
  
  <p><a href="https://www.youtube.com/watch?v=nefopNkZmB4&amp;index=3&amp;list=PL6gx4Cwl9DGAcbMi1sH6oAMk4JHw91mC_" rel="noreferrer">https://www.youtube.com/watch?v=nefopNkZmB4&amp;index=3&amp;list=PL6gx4Cwl9DGAcbMi1sH6oAMk4JHw91mC_</a></p>
  
  <p>This is my code:</p>
  
  <pre><code>    from tkinter import *
  
  
  def iCalc(source, side):
      storeObj = Frame(source, borderwidth=4, bd=4, bg="powder blue")
      storeObj.pack(side=side, expand=YES, fill=BOTH)
      return storeObj
  
  
  def button(source, side, text, command=None):
      storeObj = Button(source, text=text, command=command)
      storeObj.pack(side=side, expand=YES, fill=BOTH)
      return storeObj
  
  
  class app(Frame):
  
      def __init__(self):
          Frame.__init__(self)
          self.option_add('*Font', 'arial 20 bold')
          self.pack(expand=YES, fill=BOTH)
          self.master.title('Calculator')
  
      display = StringVar()
      Entry(self, relief=RIDGE, textvariable=display, justify='right', bd=30, bg="powder blue").pack(side=TOP, expand=YES,
                                                                                                     fill=BOTH)
  for clearBut in (["CE"], ["C"]):
      erase = iCalc(self, TOP)
  for ichar in clearBut:
      button(erase, LEFT, ichar,
             lambda storeObj=display, q=ichar: storeObj.set(''))
  
  for NumBut in ("789/", "456*", "123-", "0.+"):
      FunctionNum = iCalc(self, TOP)
  for iEquals in NumBut:
      button(FunctionNum, LEFT, iEquals,
             lambda storeObj=display, q=iEquals: storeObj.set(storeObj.get() + q))
  EqualsButton = iCalc(self, TOP)
  for iEquals in '=':
      if iEquals in "=":
          btniEquals = button(EqualsButton, LEFT, iEquals)
          btniEquals.bind('&lt;ButtonRelease-1&gt;',
                          lambda e, s=self, storeObj=display: s.calc(storeObj), '+')
  else:
      btniEquals = button(EqualsButton, LEFT, iEquals,
                          lambda storeObj=display, s=' %s ' % iEquals: storeObj.set(storeObj.get() + s))
  
  
  def calc(self, display):
      try:
          display.set(eval(display.get()))
      except:
          display.set("ERROR")
  
  if __name__ == '__main__':
      app().mainloop()
  </code></pre>
  
  <p>I am getting error:</p>
  
  <p><a href="https://i.stack.imgur.com/fvEmw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/fvEmw.png" alt="enter image description here"></a></p>
  
  <p>What am I doing wrong?</p>