title: LUA attempt to index global nil value
body: <p>I've read other answers with the same title but to no avail. My problem looks simple but I can't figure out a way to deal with. Just started LUA a few days ago. Here, it prints "nam" so there is collision. However, display.remove(apple) does not work. And the removeSelf() gives an error that says "attempt to index global 'apple' (a nil value)". The only thing I want to happen to apple at collision is for it to disappear.</p>
  
  <pre class="lang-lua prettyprint-override"><code>function appleCollision(self, event)
    if event.phase == "began" then
      print("nam")
      --display.remove( apple )
      apple:removeSelf()
      apple = nil
    end
  end
  
  
  local apple = display.newImageRect( "apple.png", 65, 85 )
  apple.x, apple.y = 460, -100
  apple.rotation = 15
  apple:addEventListener ( "collision", apple )
  apple.collision = appleCollision
  physics.addBody( apple, { density=1.0, friction=0.3, bounce=0.3 } )
  </code></pre>