Problems with $GUI_GR_REFRESH of the Graphic ctrl

#include <ButtonConstants.au3>
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>

$Form1 = GUICreate("Form1", 400, 400)
$Button1 = GUICtrlCreateButton("0", 0, 0, 400, 50)
Global $counter = 0
Local $x = 100
Local $y = 100
$MapGraphics = GUICtrlCreateGraphic($x, $y, 255, 255)
GUICtrlSetBkColor(-1, 0x000000)
GUISetState(@SW_SHOW)

While 1
    $nMsg = GUIGetMsg()
    Switch $nMsg
        Case $GUI_EVENT_CLOSE
            Exit
        Case $Button1
            $counter += 1
            GUICtrlSetData($Button1, $counter)
            $text = ''
            For $i = 0 to 65025/255
                Local $num = Random(0, 3, 1)
                For $j = 0 to 255
                    $text &= $num
                Next
            Next
            Local $mapDots = StringSplit($text, '')
            Local $x = 0
            Local $y = 0
            For $i = 1 to $mapDots[0]
                Switch Number($mapDots[$i])
                    Case 1
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_COLOR, 0x006400)
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_PIXEL, $x, $y)
                    Case 2
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_COLOR, 0xFF8000)
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_PIXEL, $x, $y)
                    Case Else
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_COLOR, 0x000000)
                        GUICtrlSetGraphic($MapGraphics, $GUI_GR_PIXEL, $x, $y)
                EndSwitch
                $x += 1
                If $x = 256 Then
                    $x = 0
                    $y += 1
                EndIf
            Next
            GUICtrlSetGraphic($MapGraphics, $GUI_GR_REFRESH)
    EndSwitch
WEnd