How to use Global variable with true-autoit-multi-threading

#NoTrayIcon ; Should add this to prevent tray icon in sub-thread.
#include 'N.au3'

global $g = NGlobal()


func task_1($l)
    $g.number = 50
endfunc

func task_2($l)

    local $msg = StringFormat('Number is %d', $g.number)
    MsgBox(0, 'Task 2 (ID: ' & NGetId() & ')', $msg)
endfunc

; main
func main()
    ; Create GUI with 3 buttons.
    GUICreate('Test 1', 300, 180)
    local $btn1 = GUICtrlCreateButton('Task 1', 20, 50, 80, 30)
    local $btn2 = GUICtrlCreateButton('Task 2', 120, 50, 80, 30)
    GUISetState(@SW_SHOW)

    $g.number = 20

    while true
        switch GUIGetMsg()
            ; close
            case -3
                exit

            ; button 1
            case $btn1

                NRun('task_1')

            ; button 2
            case $btn2
                NRun('task_2')
        endswitch
    wend
endfunc

; Execute main entry.
NMain('main')