trouble during wrapping a windows DLL

[oyster]

I am trying to write a binding for www.xcgui.com which is a GUI lib for 
windows. The initial code is on 
[https://github.com/retsyo/xcgui_nim](https://github.com/retsyo/xcgui_nim)

but I met some questions. for example 
    
    
    import ../../xcgui
    import winim/inc/[windef, wingdi]
    import winim/winstr
    import sugar
    
    proc OnClick(bHandle: var bool): cint =
        var
            sel = XC_MessageBox(nil, "炫彩界面库pText", "pCaption", 1)
        bHandle = true
        return 0
    
    proc Main() =
        #~ type ptrFnCallBack = (proc(bHandle: var bool): cint)
        #~ type ptrFnCallBack = (cint - > cint)    # sugar does not work
        #~ var fnCallBack: ptrFnCallBack = OnClick
        
        var
            bOk = XInitXCGUI()
        
        if bOk == True:
            var
                hWindow = XWnd_Create(0, 0, 300, 200, "炫彩NIM例子", 0, 
xc_window_style_default)
            
            if hWindow != nil :
                var hBtn = XBtn_Create(8, 30, 100, 20, "点我", hWindow)
                
                if hBtn != nil :
                    XEle_RegEventC( hBtn, XE_BNCLICK, cast[cstring](OnClick))
                    XWnd_ShowWindow(hWindow, 5)
                    XRunXCGUI()
                else:
                    echo "a"
            
            XExitXCGUI()
    
    if isMainModule:
        Main()
    
    
    Run

will crash if I click the button 
    
    
    SIGSEGV: Illegal storage access. (Attempt to read from nil?)
    Error: execution of an external program failed: 
'E:\my_project\nim\xcgui_\nim_\Cpp\002buttonevent\WinMain.exe '
    
    
    Run

can someone help me to fix it? thanks