Unhandled exception at KernelBase.dll in Iexplore.exe while setting a handle for plugin window

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C++

C++/CLI

browser

Being a newbie to making browser plugins, I'm making a firebreath based plugin which adds two integers. I'm debugging the plugin by attaching the source to the browser. I'm getting the above exception at this step of my PLUGINAPI.cpp I have created and built the dll of the wrapper and the c# class libraries. it's working..added the project references of these to my firebreath api project. In PLUGINAPI project>>

int PLUGINAPI::PerformAddition(int a, int b)
{
    IWrapIt *wrapIt=IWrapIt::CreateInstance();
    HWND handler=getPlugin()->AttachedWindow;
    wrapIt->SetHandle(handler);//ABOVE EXCEPTION RAISED HERE
    return wrapIt->PerformAddition(a,b);
}
In PLUGIN.cpp(window attached event)>>

 bool PLUGIN::onWindowAttached(FB::AttachedEvent *evt, FB::PluginWindow * wnd)
{
    // The window is attached; act appropriately
    AttachedWindow = wnd->get_as<fb::pluginwindowwin xmlns:fb="#unknown">()->getHWND();
    return false;
}

What am I doing wrong here? Also My debug point in c# class library is not hit even if i have set the Property of the firebreath (unmanaged c++) debugging to mixed