AddEventListener: Listener cannot be nil - lua

Question

Can someone please explain what is going wrong here? It went wrong as soon as I added the AddEventListener
newBalloon:addEventListener( "tap", pushBalloon )
Complete Code:
local composer = require( "composer" )
local scene = composer.newScene()
local physics = require( "physics" )
physics.start()
-- Configure image sheet
local positioninsheetOptions = 144.1
local sheetOptions =
{
frames =
{
{
x = 0,
y = 0,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*2,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*3,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*4,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*5,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*6,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*7,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*8,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*9,
width = 112,
height = 142
},
{
x = 0,
y = positioninsheetOptions*10,
width = 112,
height = 142
},
},
}
local objectSheet = graphics.newImageSheet( "gameObjects.png", sheetOptions )
local tapCount = 0
local platform
local tapText
local balloonsTable = {}
local leftBorder
local rightBorder
local backGroup
local mainGroup
local uiGroup
local platform
local platform2
local function createBalloon()
local randomBalloon = math.random( 10 )
local newBalloon = display.newImageRect( objectSheet, randomBalloon, 112, 142 )
if newBalloon then
table.insert( balloonsTable, newBalloon )
physics.addBody( newBalloon, "dynamic", { radius=70, bounce=0 } )
newBalloon.myName = "bigBalloon"
newBalloon.alpha = 0.75
newBalloon.gravityScale = randomBalloon/-150
newBalloon:addEventListener( "tap", pushBalloon )
end
local whereFrom = math.random( 3 )
if ( whereFrom == 1 ) then
-- From the left
newBalloon.x = 100
newBalloon.y = display.contentHeight+300
elseif ( whereFrom == 2 ) then
-- From the top
newBalloon.x = 160
newBalloon.y = display.contentHeight+300
elseif ( whereFrom == 3 ) then
-- From the right
newBalloon.x = 220
newBalloon.y = display.contentHeight+300
end
end
local function gameLoop()
-- Create new balloon
createBalloon()
-- Remove balloons which have drifted off screen
for i = #balloonsTable, 1, -1 do
local thisBalloon = balloonsTable[i]
if ( thisBalloon.x < -100 or
thisBalloon.x > display.contentWidth + 100 or
thisBalloon.y < -100 )
then
display.remove( thisBalloon )
table.remove( balloonsTable, i )
end
end
end
local function pushBalloon( event )
local tappedBalloon = event.target
-- balloon:applyLinearImpulse( 0.2, -2, balloon.x, balloon.y )
-- tapCount = tapCount + 1
-- tapText.text = tapCount
if event.phase == "began" then
tappedBalloon.gravityScale = 10
end
end
local function pushBalloon2()
-- balloon:applyLinearImpulse( 0.2, -2, balloon.x, balloon.y )
-- tapCount = tapCount + 1
-- tapText.text = tapCount
balloon2.gravityScale = 10
balloon2:applyLinearImpulse( 0.1, 0, balloon2.x, balloon2.y )
end
local function pushBalloon3()
-- balloon:applyLinearImpulse( 0.2, -2, balloon.x, balloon.y )
-- tapCount = tapCount + 1
-- tapText.text = tapCount
balloon3.gravityScale = 10
balloon3:applyLinearImpulse( -0.1, 0, balloon3.x, balloon3.y )
end
-- -----------------------------------------------------------------------------------
-- Scene event functions
-- -----------------------------------------------------------------------------------
-- create()
function scene:create( event )
local sceneGroup = self.view
-- Code here runs when the scene is first created but has not yet appeared on screen
physics.pause()
local background = display.newImageRect( "background.png", 360, 570 )
background.x = display.contentCenterX
background.y = display.contentCenterY
platform = display.newImageRect( "platform.png", 10, display.contentHeight*5 )
platform.x = -5
platform.y = 0
physics.addBody( platform, "static", { friction=0.5, bounce=0.3 } )
platform2 = display.newImageRect( "platform.png", 10, display.contentHeight*5 )
platform2.x = display.contentWidth+5
platform2.y = 0
physics.addBody( platform2, "static", { friction=0.5, bounce=0.3 } )
createBalloon()
end
-- show()
function scene:show( event )
local sceneGroup = self.view
local phase = event.phase
if ( phase == "will" ) then
-- Code here runs when the scene is still off screen (but is about to come on screen)
elseif ( phase == "did" ) then
gameLoopTimer = timer.performWithDelay( 1250, gameLoop, 0 )
-- Code here runs when the scene is entirely on screen
physics.start()
end
end
-- hide()
function scene:hide( event )
local sceneGroup = self.view
local phase = event.phase
if ( phase == "will" ) then
-- Code here runs when the scene is on screen (but is about to go off screen)
elseif ( phase == "did" ) then
-- Code here runs immediately after the scene goes entirely off screen
physics.pause()
end
end
-- destroy()
function scene:destroy( event )
local sceneGroup = self.view
-- Code here runs prior to the removal of scene's view
end
-- -----------------------------------------------------------------------------------
-- Scene event function listeners
-- -----------------------------------------------------------------------------------
scene:addEventListener( "create", scene )
scene:addEventListener( "show", scene )
scene:addEventListener( "hide", scene )
scene:addEventListener( "destroy", scene )
-- -----------------------------------------------------------------------------------
return scene
Thank you.

Answer 1

You refer to function (pushBalloon) do not exist yet when add event listener. So put definition of function above line you add event listener like that
local function pushBalloon( event )
local tappedBalloon = event.target
-- balloon:applyLinearImpulse( 0.2, -2, balloon.x, balloon.y )
-- tapCount = tapCount + 1
-- tapText.text = tapCount
if event.phase == "began" then
tappedBalloon.gravityScale = 10
end
end
...
newBalloon:addEventListener( "tap", pushBalloon )

Answer 2

At the point in the file where you are adding the listener, pushBalloon() is not defined. You should add a forward declaration earlier in the file:
local scene = ...
local pushBalloon
...
-- createBalloon() and other existing code
...
pushBalloon = function ( event )
-- your function code
end