How to solve error Attempt to perform arithmetic on field 'x' (a nil value)

Getting Started Newbie Questions
#1

I have two Runtimes. And when I go to another composer Scene… I get the error

Attempt to perform arithmetic on field ‘x’ (a nil value)

&nbsp;&nbsp;&nbsp; -- Circle-based collision detection local function hasCollidedCircle( obj1, obj2 ) &nbsp;&nbsp;&nbsp; if ( obj1 == nil ) then&nbsp; -- Make sure the first object exists &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return false &nbsp;&nbsp;&nbsp; end &nbsp;&nbsp;&nbsp; if ( obj2 == nil ) then&nbsp; -- Make sure the other object exists &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return false &nbsp;&nbsp;&nbsp; end &nbsp;&nbsp;&nbsp; local dx = obj1.x - obj2.x --// HERE IS THE ERROR &nbsp;&nbsp;&nbsp; local dy = obj1.y - obj2.y &nbsp;&nbsp;&nbsp; local distance = math.sqrt( dx\*dx + dy\*dy ) &nbsp;&nbsp;&nbsp; local objectSize = (obj2.contentWidth/2) + (obj1.contentWidth/2) &nbsp;&nbsp;&nbsp; if ( distance \< objectSize ) then &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return true &nbsp;&nbsp;&nbsp; end &nbsp;&nbsp;&nbsp; return false end hasCollidedCircle( obj1, obj2 )

I remove the Runtime here

&nbsp;&nbsp;&nbsp; local buttonBack = display.newImageRect(sceneGroup, "images/backButton.png", 150, 150) &nbsp;&nbsp;&nbsp; buttonBack.x = 100 &nbsp;&nbsp;&nbsp; buttonBack.y = 60 &nbsp;&nbsp;&nbsp; local function buttonBackListener() &nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;Runtime:removeEventListener("enterFrame", explosion ) &nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;Runtime:removeEventListener("touch", screenTouched) &nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;composer.gotoScene("menu3") &nbsp;&nbsp;&nbsp; end &nbsp;&nbsp;&nbsp; buttonBack:addEventListener("tap", buttonBackListener)

When can I be doing wrong.

Thanks for your help

and Happy new Year 2016