Попытка вызвать виртуальный метод byte[] java.lang.String.getBytes() для нулевой ссылки на объект в React Native Expo.

У меня есть приложение React Native Expo, но оно продолжает выдавать эту ошибку:

Attempt to invoke virtual method 'byte[] java.lang.String.getBytes()' on a null object reference
at node_modules\react-native\Libraries\BatchedBridge\NativeModules.js:103:50 in promiseMethodWrapper
at node_modules\@unimodules\react-native-adapter\build\NativeModulesProxy.native.js:15:23 in moduleName.methodInfo.name
at node_modules\expo-file-system\build\FileSystem.js:50:17 in writeAsStringAsync
at node_modules\regenerator-runtime\runtime.js:63:36 in tryCatch
at node_modules\regenerator-runtime\runtime.js:293:29 in invoke
at node_modules\regenerator-runtime\runtime.js:63:36 in tryCatch
at node_modules\regenerator-runtime\runtime.js:154:27 in invoke
at node_modules\regenerator-runtime\runtime.js:189:16 in PromiseImpl$argument_0
at node_modules\react-native\node_modules\promise\setimmediate\core.js:45:6 in tryCallTwo
at node_modules\react-native\node_modules\promise\setimmediate\core.js:200:22 in doResolve
at node_modules\react-native\node_modules\promise\setimmediate\core.js:66:11 in Promise
at node_modules\regenerator-runtime\runtime.js:188:15 in callInvokeWithMethodAndArg
at node_modules\regenerator-runtime\runtime.js:211:38 in enqueue
at node_modules\regenerator-runtime\runtime.js:238:8 in exports.async
at node_modules\expo-file-system\build\FileSystem.js:46:7 in writeAsStringAsync
at screens\Export.js:74:12 in fetch.then.then$argument_0
at node_modules\regenerator-runtime\runtime.js:63:36 in tryCatch
at node_modules\regenerator-runtime\runtime.js:293:29 in invoke
at node_modules\regenerator-runtime\runtime.js:63:36 in tryCatch
at node_modules\regenerator-runtime\runtime.js:154:27 in invoke
at node_modules\regenerator-runtime\runtime.js:189:16 in PromiseImpl$argument_0
at node_modules\react-native\node_modules\promise\setimmediate\core.js:45:6 in tryCallTwo
at node_modules\react-native\node_modules\promise\setimmediate\core.js:200:22 in doResolve
at node_modules\react-native\node_modules\promise\setimmediate\core.js:66:11 in Promise
at node_modules\regenerator-runtime\runtime.js:188:15 in callInvokeWithMethodAndArg
at node_modules\regenerator-runtime\runtime.js:211:38 in enqueue
at node_modules\regenerator-runtime\runtime.js:238:8 in exports.async
at screens\Export.js:65:14 in exportReport
at node_modules\react-native\node_modules\promise\setimmediate\core.js:37:13 in tryCallOne
at node_modules\react-native\node_modules\promise\setimmediate\core.js:123:24 in setImmediate$argument_0
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:130:14 in _callTimer
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:181:14 in _callImmediatesPass
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:441:30 in callImmediates
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:387:6 in __callImmediates
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:135:6 in __guard$argument_0
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:364:10 in __guard
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:134:4 in flushedQueue
at [native code]:null in flushedQueue
at [native code]:null in invokeCallbackAndReturnFlushedQueue

Это код, который создает эту ошибку. Кажется, это ошибка Java, хотя я использую реагирующую нативную выставку.

  const exportReport = async () => {
    if (validate()) {
      setLoading(true);
      let token = await SecureStore.getItemAsync("token");
      return await fetch("https://notrealapi.herokuapp.com/api/export", {
        method: "POST",
        headers: {
          "content-type": "application/json",
          Authorization: `JWT ${JSON.parse(token)}`,
        },
        body: JSON.stringify({ note: note, mission: value }),
      })
        .then((res) => res.json())
        .then(async (json) => {
          if (json.hasOwnProperty("msg")) {
            Alert.alert(json.msg);
          } else {
            setLoading(false);
            console.log(json);
            setReportUrl("data:application/pdf;base64," + json.pdf);
            console.log(reportUrl);
            const base64Code = reportUrl.split(
              "data:application/pdf;base64,"
            )[1];
            const filename = FileSystem.documentDirectory + "report.pdf";
            console.log(base64Code);
            await FileSystem.writeAsStringAsync(filename, base64Code, {
              encoding: FileSystem.EncodingType.Base64,
            });
            const _mediaResult = await MediaLibrary.saveToLibraryAsync(
              filename
            );
            await Sharing.shareAsync(filename);
          }
        })
        .catch((error) => console.error(error));
    }
  };

Я получаю строку из серверной части, затем делаю из этой строки URL-адрес данных, а затем сохраняю этот URL-адрес данных в формате pdf на телефоне пользователя. Это работает после нескольких попыток.

Кроме того, вот как серверная часть выдает строку (с использованием Django):

pdfStr = pdf.output(dest="S").encode("latin-1")
encodedPdfStr = base64.b64encode(pdfStr)
return Response({"pdf": encodedPdfStr})

Мы будем признательны за любую помощь.

Спасибо.