我如何在以下代码的主要活动中使用“客户端”？

Question

因此，我需要在Android上开发一个Client应用程序，该应用程序将与服务器通信并使用OPCUA协议访问服务器的地址空间。

我有一个如下所示的异步任务，它构成了连接。

我无法在主要活动中访问客户端的属性。给我一个空指针异常。请引导我。

PS：OOP和Android Ofcourse的初学者。

主要活动

  protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        connectButton = findViewById(R.id.connectButton);
        textView = findViewById(R.id.textView);

        ConnectAndReadTask task = new ConnectAndReadTask();

        task.execute();

        try{
            System.out.println("NSI" +client.getAddressSpace().getNamespaceTable().getIndex("opc.tcp://192.168.4.1:4840")); //getting a nullpointer here
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

异步任务

public class ConnectAndReadTask extends AsyncTask<Void, Void, String> {

        public ConnectAndReadTask() {
        }

        @Override
        public String doInBackground(Void... params) {
            UaClient client = null;
            String result = null;
            try {
                System.out.println("Connecting...");
                client = new UaClient("opc.tcp://192.168.4.1:4840");
                //System.out.println("Connected");

                ApplicationDescription appDescription = new ApplicationDescription();
                appDescription.setApplicationName(new LocalizedText("SimpleAndroidClient",
                        Locale.ENGLISH));
                String android_id = Settings.Secure.getString(
                        getApplicationContext().getContentResolver(),
                        Settings.Secure.ANDROID_ID);

                appDescription.setApplicationUri("urn:" + android_id + ":UA:SimpleAndroidClient");
                appDescription.setProductUri("urn:prosysopc.com:UA:SimpleAndroidClient");
                appDescription.setApplicationType(ApplicationType.Client);

                PkiFileBasedCertificateValidator validator = new PkiFileBasedCertificateValidator(
                        getFilesDir().getPath() + "/PKI/CA");
                validator.setValidationListener(new CertificateValidationListener() {
                    @Override
                    public PkiFileBasedCertificateValidator.ValidationResult onValidate(
                            Cert cert, ApplicationDescription applicationDescription,
                            EnumSet<PkiFileBasedCertificateValidator.CertificateCheck> enumSet) {
                        return PkiFileBasedCertificateValidator.ValidationResult.AcceptPermanently;
                    }
                });
                client.setCertificateValidator(validator);

                ApplicationIdentity identity = ApplicationIdentity.loadOrCreateCertificate(
                        appDescription, "Sample Organisation", "opcua",
                        new File(validator.getBaseDir(), "private"), true);

                identity.setApplicationDescription(appDescription);
                client.setApplicationIdentity(identity);

                client.setTimeout(60000);
                client.setSecurityMode(SecurityMode.NONE);
                client.setUserIdentity(new UserIdentity());
                System.out.println("Lets connect");
                client.connect();
} catch (Exception e) {
                result = e.toString();
            }
           // return result;
            return client;

     textView.setText(s);
        protected void onPostExecute(UaClient client) {
            textView.setText(client.toString());
        }

}
}