Error Calling method on NPObject - Firefox Plugin

I am having a piece of visual C++ code (firebreath) that retrieves data from a visual C# application. The communication works fine for negative cases(returns value without out parameter) but in positive case(returns value with out parameter) the following error is shown.

Error: Error calling method on NPObject!

I guess the problem is in the out parameter of Visual C# application. Can anyone help me in this?? Kindly use the below code for reference.

ngContent.js: (js to call firebreath function)

function GetDetails(param1, param2) {
  try {
    return document.getElementById("nGCall").ReturnDetails(param1, param2);
  }
  catch (e) {
    alert("Exception Occured " + e.message);
  }
};

nGAPI.cpp: (Firebreath function to call C# application)

FB::VariantList nGAPI::ReturnDetails(std::wstring& param1, std::wstring& param2)
{
  try {
    InetGuardNPAPI *CSharpInterface = NULL;

    //Open interface to C#
    CoInitialize(NULL);
    HRESULT hr = CoCreateInstance(CLSID_netGuardIEBHO, NULL, CLSCTX_INPROC_SERVER, IID_InetGuardNPAPI, reinterpret_cast<void**>(&CSharpInterface));

    BSTR out1= NULL;
    BSTR out2= NULL;
    BSTR out3= NULL;
    BSTR out4= NULL;

    int returns = CSharpInterface->NPGetDetails(param1.c_str(), param2.c_str(), &out1, &out2, &out3, &out4);

    if (out1 != NULL && out2 != NULL) {
      return FB::variant_list_of(out1)(out2)(out3)(out4);
    } else {
      return FB::variant_list_of();
    }
  } catch (...) {
    MessageBoxW(NULL, L"Exception occured.", L"NG", NULL);

    return FB::variant_list_of();
  }

nGHost.cs: (Visual C# application)

public int NPGetDetails(string param1, string param2, out string out1, out string out2, out string out3, out string out4)
{
  int retValue = 0;
  out1 = null;
  out2 = null;
  out3 = null;
  out4 = null;

  bool userIdentified = IdentifyUser(ngDB, out ngdata);

  if (!userIdentified)
    return retValue;

  try {
    out1 = ngdata.abc;
    out2 = ngdata.def;
    out3 = ngdata.ghi;
    out4 = ngdata.jkl;
    retValue = 1;
  } catch (Exception ex) {
    MessageBox.Show(ex.Message);
  }

  return retValue;
}

Thanks in advance.

The error you're getting indicates that an exception was thrown. Unfortunately the browsers stopped exposing exceptions when they switched to out of process plugins. I'd recommend attaching a debugger and stepping through to see what is actually happening; alternately, add some logging.