Exception when trying to convert an IInputStream to a Platform::String using DataReader loop

Question

Hi,

I am trying to convert an inputstream from a zip file holding an utf8 string into a Platform::String to load it Inside the xml parser, so I wrote the following code :

Platform::String^ Container::ConvertUTF8StreamToPString(IInputStream^ inputStream, size_t BUF_SIZE /*= 4096*/)
{
	Platform::String^ str = nullptr;

	std::wstring wstr;
	using Converter = std::wstring_convert<std::codecvt_utf8<wchar_t>>;
	Converter converter;

	if (inputStream != nullptr)
	{
		Platform::WriteOnlyArray<unsigned char, 1U>^ buf = nullptr;
		DataReader^ dataReader = ref new DataReader(inputStream);
		dataReader->UnicodeEncoding = UnicodeEncoding::Utf8;

		
		int numRead = 0;

		do
		{
			auto opLoadData = create_task(dataReader->LoadAsync(BUF_SIZE));
			opLoadData.then([dataReader, &converter, &wstr, &numRead](unsigned int numBytesLoaded) {

				numRead = numBytesLoaded;
				if (numRead > 0)
				{
					Platform::WriteOnlyArray<unsigned char, 1U>^ buf = ref new Platform::Array<unsigned char, 1U>(numRead);
					dataReader->ReadBytes(buf);
					wstr.append(converter.from_bytes(reinterpret_cast<char*>(buf->Data), reinterpret_cast<char*>(buf->Data + numRead)));
				}

			}).wait();

		} while (numRead > 0);
	}

	str = ref new Platform::String(wstr.c_str());

	return str;
}

The first loop works fine but the second one where there is nothing more to read throw an null pointer exception, how can I fix this code ?

Thanks