NullPointerException при подключении

Я думаю, что излишне будет писать код активити. Класс был успешно испытан на консоли Java.

Листинг:

public class ConnectorZabbix {
    public String goIntoZabbix(String postJsonFile) throws IOException {
        // METHOD NEED GSON-FILE OF TYPE STRING
        URL url = null;
        HttpURLConnection httpURLConnection = null;
        DataInputStream input;
        DataOutputStream output = null;
        String response = null;
        try {
            url = new URL(LoginActivity.zabbixServerUrl.toString());
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }

        httpURLConnection = (HttpURLConnection) url.openConnection();
        httpURLConnection.setDoInput(true);
        httpURLConnection.setDoOutput(true);
        try {
            httpURLConnection.setRequestMethod("POST");
        } catch (ProtocolException e) {
            e.printStackTrace();
        }
        httpURLConnection.setRequestProperty("Content-Type", "application/json");
        httpURLConnection.connect();
        output = new DataOutputStream(httpURLConnection.getOutputStream());
        output.writeBytes(postJsonFile);
        output.flush();
        output.close();
        input = new DataInputStream(httpURLConnection.getInputStream());

        if (null != (response = input.readUTF())) {
            boolean asfasf = false;
        }
        input.close();
        return response;
    }

}

Логи на месте ошибки:

03-16 23:04:47.343 1777-1777/myzabbix.zaynukov.sadrutdin.com.zabbixapiimplementation E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                                       Process: myzabbix.zaynukov.sadrutdin.com.zabbixapiimplementation, PID: 1777
                                                                                                       Theme: themes:{}
                                                                                                       java.lang.NullPointerException: Attempt to invoke virtual method 'java.net.URLConnection java.net.URL.openConnection()' on a null object reference
                                                                                                           at myzabbix.zaynukov.sadrutdin.com.zabbixapiimplementation.objects.ConnectorZabbix.goIntoZabbix(ConnectorZabbix.java:34)
                                                                                                           at myzabbix.zaynukov.sadrutdin.com.zabbixapiimplementation.LoginActivity$1.onClick(LoginActivity.java:57)
                                                                                                           at android.view.View.performClick(View.java:5204)
                                                                                                           at android.view.View$PerformClick.run(View.java:21158)
                                                                                                           at android.os.Handler.handleCallback(Handler.java:739)
                                                                                                           at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                                           at android.os.Looper.loop(Looper.java:148)
                                                                                                           at android.app.ActivityThread.main(ActivityThread.java:5461)
                                                                                                           at java.lang.reflect.Method.invoke(Native Method)
                                                                                                           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Ошибку показывает здесь:

httpURLConnection = (HttpURLConnection) url.openConnection();

В манифест был добавлен:

<uses-permission android:name="android.permission.INTERNET" />