title: Attempt to index upvalue which is a nil value
body: <p>The error "attempt to index upvalue 'askUser'(a nil value)" always shows up, I had to make the statement to a comment to run the app, how do i fix it?</p>
  
  <pre><code>function restartLvl()  
          for i = 1, #balloonTexts do
          display.remove(balloonTexts[i])
          print ("restart level")
      end
      score.text = '0'
      ballRemain.text = '3'
      balloonText = {}
      createBalloons(1, 3)
      if (askUser.isVisible == true) then --this is where the error occured
      askUser.isVisible = false
      end
      if (yesBtn.isVisible == true) then
      yesBtn.isVisible = false
      end
      if (noBtn.isVisible == true) then
      noBtn.isVisible = false
      end
  
      print("time from start: ", (system.getTimer()-gameTime))
      print('send mail')
      sendMail()
      restartBtn:removeEventListener('tap', restartLvl)
  
  end
  </code></pre>