Lua/Corona sdk error - lua

Question

I'm having this issue, and I can't find a solution:
"level1.lua:161 attempt to index global 'crate' (a nil value)"
It happens when it should change the crate position on line 161, but "centerX + (centerX * event.xGravity" does not return nil, cause the "textMessage" show the correct value.
Code below
-----------------------------------------------------------------------------------------
--
-- level1.lua
--
-----------------------------------------------------------------------------------------
local storyboard = require( "storyboard" )
local scene = storyboard.newScene()
-- include Corona's "physics" library
local physics = require "physics"
physics.start(); physics.pause()
--------------------------------------------
-- Sounds
local shakeSound = audio.loadSound ("shake.mp3")
-- Display, metrics stuff
local centerX = display.contentWidth / 2
local centerY = display.contentHeight / 2
-- forward declarations and other locals
local screenW, screenH, halfW = display.contentWidth, display.contentHeight, display.contentWidth*0.5
-----------------------------------------------------------------------------------------
-- BEGINNING OF YOUR IMPLEMENTATION
--
-- NOTE: Code outside of listener functions (below) will only be executed once,
-- unless storyboard.removeScene() is called.
--
-----------------------------------------------------------------------------------------
-- Text parameters
local labelx = 50
local x = 220
local y = 95
local fontSize = 24
local frameUpdate = false -- used to update our Text Color (once per frame)
local xglabel = display.newText( "gravity x = ", labelx, y, native.systemFont, fontSize )
xglabel:setTextColor(255,255,255)
local textMessage = function( str, location, scrTime, size, color, font )
local x, t
size = tonumber(size) or 24
color = color or {255, 255, 255}
font = font or "Helvetica"
-- Determine where to position the text on the screen
if "string" == type(location) then
if "Top" == location then
x = display.contentHeight/4
elseif "Bottom" == location then
x = (display.contentHeight/4)*3
else
-- Assume middle location
x = display.contentHeight/2
end
else
-- Assume it's a number -- default to Middle if not
x = tonumber(location) or display.contentHeight/2
end
scrTime = (tonumber(scrTime) or 3) * 1000 -- default to 3 seconds (3000) if no time given
t = display.newText(str, 0, 0, font, size )
t.x = display.contentWidth/2
t.y = x
t:setTextColor( color[1], color[2], color[3] )
-- Time of 0 = keeps on screen forever (unless removed by calling routine)
--
if scrTime ~= 0 then
-- Function called after screen delay to fade out and remove text message object
local textMsgTimerEnd = function()
transition.to( t, {time = 500, alpha = 0},
function() t.removeSelf() end )
end
-- Keep the message on the screen for the specified time delay
timer.performWithDelay( scrTime, textMsgTimerEnd )
end
return t -- return our text object in case it's needed
end -- textMessage()
-- Called when the scene's view does not exist:
function scene:createScene( event )
local group = self.view
-- create a grey rectangle as the backdrop
local background = display.newRect( 0, 0, screenW, screenH )
background:setFillColor( 128 )
-- make a crate (off-screen), position it, and rotate slightly
local crate = display.newImage('crate.png') --display.newImageRect( "crate.png", 90, 90 )
crate.x = centerX
crate.y = centerY
crate.rotation = 15
-- add physics to the crate
physics.addBody( crate, { density=1.0, friction=0.3, bounce=0.3 } )
-- create a grass object and add physics (with custom shape)
local grass = display.newImageRect( "grass.png", screenW, 82 )
grass:setReferencePoint( display.BottomLeftReferencePoint )
grass.x, grass.y = 0, display.contentHeight
-- define a shape that's slightly shorter than image bounds (set draw mode to "hybrid" or "debug" to see)
local grassShape = { -halfW,-34, halfW,-34, halfW,34, -halfW,34 }
physics.addBody( grass, "static", { friction=0.3, shape=grassShape } )
-- all display objects must be inserted into group
group:insert( background )
group:insert( grass)
group:insert( crate )
end
-- Called immediately after scene has moved onscreen:
function scene:enterScene( event )
local group = self.view
physics.start()
end
-- Called when scene is about to move offscreen:
function scene:exitScene( event )
local group = self.view
physics.stop()
end
-- If scene's view is removed, scene:destroyScene() will be called just prior to:
function scene:destroyScene( event )
local group = self.view
package.loaded[physics] = nil
physics = nil
end
-- Accelerometer
local function onAccelerate( event )
-- Move our object based on the accelerator values --
textMessage( tostring(centerX + (centerX * event.xGravity)), "naosei", 0.5, 12, {255, 255, 0} )
crate.x = centerX + (centerX * event.xGravity)
crate.y = centerY + (centerY * event.yGravity * -1)
-- sound beep if Shake'n
if event.isShake == true then
textMessage( "Shake!", "Top", 3, 52, {255, 255, 0} )
audio.play( shakeSound )
end
end
-----------------------------------------------------------------------------------------
-- END OF YOUR IMPLEMENTATION
-----------------------------------------------------------------------------------------
-- Add runtime listeners
Runtime:addEventListener ("accelerometer", onAccelerate);
-- Add scene listeners
-- "createScene" event is dispatched if scene's view does not exist
scene:addEventListener( "createScene", scene )
-- "enterScene" event is dispatched whenever scene transition has finished
scene:addEventListener( "enterScene", scene )
-- "exitScene" event is dispatched whenever before next scene's transition begins
scene:addEventListener( "exitScene", scene )
-- "destroyScene" event is dispatched before view is unloaded, which can be
-- automatically unloaded in low memory situations, or explicitly via a call to
-- storyboard.purgeScene() or storyboard.removeScene().
scene:addEventListener( "destroyScene", scene )
-----------------------------------------------------------------------------------------
return scene

Answer 1

"level1.lua:161 attempt to index global 'crate' (a nil value)"
Because there is no crate variable in scope on line 161.
You do have a variable named crate inside of createScene but it's local, so it's only visible within that function. Line 161 is a different function (onAccelerate), there is no local crate variable in scope there, so Lua looks for a global (_G['crate']), gets back nil, and you try to index that. Hence the error.
Easiest fix: remove the keyword local from the crate on line 108, so you create a global. It's not pretty, but it should work.

Answer 2

I would avoid using global variables because of memory leak issues.
What I do is define the display object names in the root scope and then set them from within the createScene function. I think this helps lua to do garbage collection.
local background, crate
function scene:createScene( event )
local group = self.view
local background = display.newRect( 0, 0, screenW, screenH )
local crate = display.newImage('crate.png')
group:insert( background )
group:insert( crate )
end