Android setText（）方法导致活动崩溃

Question

我是Android新手，我在填写textView以显示错误消息方面遇到了问题。 在RSSReader服务中，在doInBackground（）方法中，当它无法从NewsActivity给出的url中获取数据时，它应该在TextView errorMessage中设置一些消息，并且仅当给定的url根本不是url时它才有效，否则活动崩溃了。

我不明白为什么方法setText（）并不总是有效。

这是ReadRss服务类

public class ReadRss extends AsyncTask<Void,Void,Void> {

Context context;
ProgressDialog progressDialog;
String address;
URL url;
ArrayList<FeedItem> feedItems;
RecyclerView recyclerView;
Dialog dialog;
NewsActivity newsActivity;


public ReadRss(Context context, RecyclerView recyclerView, String address, NewsActivity newsActivity) {
    this.context = context;
    this.recyclerView = recyclerView;
    this.address = address;
    this.newsActivity = newsActivity;
    progressDialog = new ProgressDialog(context);
    progressDialog.setMessage("Loading");
}

@Override
protected void onPreExecute() { //prima di caricare i dati mostro il dialog

    progressDialog.show();
    super.onPreExecute();

}

@Override
protected void onPostExecute(Void aVoid) {
    super.onPostExecute(aVoid);
    progressDialog.dismiss();
    Log.e("POST EXECUTE", "E' ENTRATO");

    MyAdapter adapter = new MyAdapter(feedItems, context);
    recyclerView.setLayoutManager(new LinearLayoutManager(context));
    recyclerView.setAdapter(adapter);

}

@Override
protected Void doInBackground(Void... voids) {

    if(getData()!=null){

        processXml(getData());
    }else {

        Log.e("DET DATA NULL", "E' ENTRATO");
        feedItems = new ArrayList<>();
        newsActivity.setErrorMessage("Invalid Url, Please delete this feed and create one with a valid url");
    }

    return null;
}



// In this method we will process Rss feed  document we downloaded to parse useful information from it
private void processXml(Document data) {
    if (data != null) {

        feedItems = new ArrayList<>();
        Element root = data.getDocumentElement();
        Node channel = root.getChildNodes().item(1);
        NodeList items = channel.getChildNodes();
        for (int i = 0; i < items.getLength(); i++) {
            Node cureentchild = items.item(i);
            if (cureentchild.getNodeName().equalsIgnoreCase("item")) {
                FeedItem item = new FeedItem();
                NodeList itemchilds = cureentchild.getChildNodes();
                for (int j = 0; j < itemchilds.getLength(); j++) {
                    Node cureent = itemchilds.item(j);
                    if (cureent.getNodeName().equalsIgnoreCase("title")) {
                        item.setTitle(cureent.getTextContent());
                    }else if (cureent.getNodeName().equalsIgnoreCase("pubDate")) {
                        item.setPubDate(cureent.getTextContent());
                    } else if (cureent.getNodeName().equalsIgnoreCase("link")) {
                        item.setLink(cureent.getTextContent());
                    }
                }
                feedItems.add(item);
            }
        }
    }else{

        feedItems = new ArrayList<>();
    }
}

public  Document getData(){
    try {
        url = new URL(address);

        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        InputStream inputStream = connection.getInputStream();
        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
        Document xmlDocument = documentBuilder.parse(inputStream);

        return xmlDocument;

    } catch (MalformedURLException e) {

        e.printStackTrace();
        return null;
    } catch (IOException e) {

        e.printStackTrace();
        return null;
    } catch (ParserConfigurationException e) {

        e.printStackTrace();
        return null;
    } catch (SAXException e) {

        e.printStackTrace();
        return null;
    }
}
}

这个NewsActivity Class

public class NewsActivity extends AppCompatActivity {

RecyclerView recyclerView;
String url;
TextView errorMessage;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_news);

    Bundle bundle = getIntent().getExtras();

    url = bundle.getString("Url");

    recyclerView = findViewById(R.id.recyclerViewNewsActivity);
    errorMessage = findViewById(R.id.error_message);

    ReadRss readRss = new ReadRss(this, recyclerView,url, this);
    readRss.execute();
}

public void setErrorMessage(String errorMessage){

    this.errorMessage.setText(errorMessage);
}

}

我希望我很清楚，谢谢你的帮助