Problem with Callbacks's first start

Hello guys, first post here.

I'm a noob in scripting.

In a script I'm trying to write, I wanted to show in a listbox the objects's name of the current selection, but the first time I launch it it doesn't work (got this error: "Unknown property: "selectedobjs" in undefined"), then the next times it works.
What did I do wrong?

Thank you in advance guys

function SetupDropDownList = (

sel_names_array= for obj in selection collect obj.name

tmpArr = for grp in selection where isGroupHead grp collect grp
numgroups = tmpArr.count as string
numobjs = (sel_names_array.count - tmpArr.count) as string

info_array=#(sel_names_array,numobjs,numgroups)
return info_array
)

function RefreshList = (

items_array = SetupDropDownList()

MyRollout.selectedobjs.items = items_array[1]
MyRollout.nselectedobjs.text = items_array[2]
MyRollout.nselectedgrps.text = items_array[3]

)

rollout MyRollout "MyRollout"

(

group "Export List" (

listbox selectedobjs

label objtext "Objects Selected:" across:2 align:#left offset:[30,0]
label nselectedobjs "objects"
label grptext "Groups Selected" across:2 align:#left offset:[30,0]
label nselectedgrps "groups"

)

)

createDialog MyRollout 200 300

try(callbacks.addScript #selectionSetChanged "RefreshList()")catch()