Navigation Controls problem

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I have a problem.During navigation from one page to another and coming back to the previous page,the controls(for eg:buttons) in the page is not working.I even can’t navigate to the next poster screen.What i did was that i combined the posterscreen template with gridscreen template.When i click an item in the gridscreen ,it will redirect to posterscreen.But it is not working.Help please?
Grid.brs

Function showGridScreen(screen As Object, gridstyle as string) As string
    poster=CreateObject("roPosterScreen")
     port = CreateObject("roMessagePort")
    poster.SetMessagePort(port)
    poster.SetListStyle("arced-landscape")
    print "enter showGridScreen"
    categoryList = getgridCategoryList()
    categoryList[0] = "GridStyle: " + gridstyle
    screen.setupLists(categoryList.count())
    screen.SetListNames(categoryList)
    StyleButtons = getGridControlButtons()
    screen.SetContentList(0, StyleButtons)
    for i = 1 to categoryList.count()-1
        screen.SetContentList(i, getgridShowsForCategoryItem(categoryList[i]))
    end for
    screen.Show()

    while true
        print "Waiting for message"
        msg = wait(0, screen.GetMessagePort())
        if type(msg) = "roGridScreenEvent" then
            if msg.isListItemFocused() then
                print"list item focused | current show = "; msg.GetIndex()
            else if msg.isListItemSelected() then
                row = msg.GetIndex()
                selection = msg.getData()
                print "list item selected row= "; row; " selection= "; selection
                value = showPosterScreen(poster)
            else if msg.isScreenClosed() then
                return ""
            end if
        end If
    end while

End Function

Poster.brs

Function showPosterScreen(screen As Object) As Integer
   
    categoryList = getCategoryList()
    screen.SetListNames(categoryList)
    screen.SetContentList(getShowsForCategoryItem(categoryList[0]))
    screen.Show()

    while true
        msg = wait(0, screen.GetMessagePort())
        if type(msg) = "roPosterScreenEvent" then
            print "showPosterScreen | msg = "; msg.GetMessage() " | index = "; msg.GetIndex()
            if msg.isListFocused() then
                'get the list of shows for the currently selected item
                screen.SetContentList(getShowsForCategoryItem(categoryList[msg.GetIndex()]))
                print "list focused | current category = "; msg.GetIndex()
            else if msg.isListItemFocused() then
                print"list item focused | current show = "; msg.GetIndex()
            else if msg.isListItemSelected() then
                print "list item selected | current show = "; msg.GetIndex() 
                VideoPlayer()
                'if you had a list of shows, the index of the current item 
                'is probably the right show, so you'd do something like this
                'm.curShow = displayShowDetailScreen(showList[msg.GetIndex()])
            else if msg.isScreenClosed() then
                return -1
            end if
        end If
    end while

End Function