[Corona SDK] How to detect the collision for physical objects - using Global Collision

For detection of collision, we can use Local Collision or Global Collision.
Below is the usage of Global Collision:

local main = display.newImage( "mainRole.png", 160, 240 )
physics.addBody( main, { density = 1.0, friction = 0.3, bounce = 0.2 } )
main.myName = "mainRole"

local fruit1 = display.newImage( "fruit.png", 100, 120 )
physics.addBody( fruit1, { density = 1.0, friction = 0.3, bounce = 0.2 } )
fruit1.myName = "fruit1"

local fruit2 = display.newImage( "fruit.png", 300, 220 )
physics.addBody( fruit2, { density = 1.0, friction = 0.3, bounce = 0.2 } )
fruit2.myName = "fruit2"

local function onGlobalCollision( event )

    if ( event.phase == "began" ) then        
        if((event.object1.myName=="mainRole" and event.object2.myName=="fruit1") or (event.object1.myName=="fruit1 and event.object2.myName=="mainRole")) then
         print( "began: " .. event.object1.myName .. " and " .. event.object2.myName )         
         if(fruit1 ~= nil) then
          print("Remove fruit1")
          fruit1:removeSelf( )
    fruit1 = nil
         end
        elseif((event.object1.myName=="mainRole" and event.object2.myName=="fruit2") or (event.object1.myName=="fruit2" and event.object2.myName=="mainRole")) then
         print( "began: " .. event.object1.myName .. " and " .. event.object2.myName )         
         if(fruit2 ~= nil) then
          print("Remove fruit2")
          fruit2:removeSelf( )
    fruit2 = nil
         end
        end

    elseif ( event.phase == "ended" ) then
        print( "ended: " .. event.object1.myName .. " and " .. event.object2.myName )
    end
end

Runtime:addEventListener( "collision", onGlobalCollision )

We add the specific name for each object.
When the collision happens, we can know which two objects collide with each other by verify the object names.

However, when we run the codes shown above, we may find that sometimes Runtime error will happen.
In executing fruit1:removeSelf( ), it will tell you that fruit1 is nil.
It is strange since we do check if(fruit1 ~= nil).
For fruit2, the problem is similar.

From the log message, the code print( "began: " .. event.object1.myName .. " and " .. event.object2.myName ) may run few times, and then execute print("Remove fruit1") after that.
It seems that this function is multi-entry.
A multi-entry function without critical section mechanism to protect it is liable to corrupt the data.
To solve this problem, we can make the removal of objects to other place, like the following codes:

local removeFruit1 = false
local removeFruit2 = false
local main = display.newImage( "mainRole.png", 160, 240 )
physics.addBody( main, { density = 1.0, friction = 0.3, bounce = 0.2 } )
main.myName = "mainRole"

local fruit1 = display.newImage( "fruit.png", 100, 120 )
physics.addBody( fruit1, { density = 1.0, friction = 0.3, bounce = 0.2 } )
fruit1.myName = "fruit1"

local fruit2 = display.newImage( "fruit.png", 300, 220 )
physics.addBody( fruit2, { density = 1.0, friction = 0.3, bounce = 0.2 } )
fruit2.myName = "fruit2"

local function onGlobalCollision( event )

    if ( event.phase == "began" ) then        
        if((event.object1.myName=="mainRole" and event.object2.myName=="fruit1") or (event.object1.myName=="fruit1 and event.object2.myName=="mainRole")) then
         print( "began: " .. event.object1.myName .. " and " .. event.object2.myName )         
         removeFruit1 = true
        elseif((event.object1.myName=="mainRole" and event.object2.myName=="fruit2") or (event.object1.myName=="fruit2" and event.object2.myName=="mainRole")) then
         print( "began: " .. event.object1.myName .. " and " .. event.object2.myName )         
         removeFruit2 = true
        end

    elseif ( event.phase == "ended" ) then
        print( "ended: " .. event.object1.myName .. " and " .. event.object2.myName )
    end
end

Runtime:addEventListener( "collision", onGlobalCollision )

local function removeAction()
 if(removeFruit1) then
   if(fruit1 ~= nil) then
      print("Remove fruit1")
      fruit1:removeSelf( )
      fruit1 = nil
      removeFruit1 = false
   end
 end
 if(removeFruit2) then
   if(fruit2 ~= nil) then
     print("Remove fruit2")
     fruit2:removeSelf( )
     fruit2 = nil
     removeFruit2 = false
   end
 end 
end
 
timer.performWithDelay( 50, removeAction,-1 )