android asynHttp下载PDF文件时报空指针?报错

手机端下载服务端的pdf文件，代码如下：

// PDF文件下载
			String[] allowedContentTypes = new String[] { "application/pdf",
					"image/png", "image/jpeg" };
			HttpUtil.get(url, params, new BinaryHttpResponseHandler(
					allowedContentTypes) {
				@Override
				public void onSuccess(byte[] binaryData) {
					super.onSuccess(binaryData);
					Log.i("-----pdf onSuccess-------", binaryData.toString());
				}

				@Override
				public void onFailure(Throwable error, String content) {
					super.onFailure(error, content);
					Log.i("-----pdf error-------", content);
				}
			});

请问这是什么原因呢？求指教