I can not get some Strings from strings.xml

I want to get a String that is in the strings.xml file, but always gives the following error:

 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.macave.rastreador/com.macave.rastreador.AjudaActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2246)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2410)
                                                       at android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313)
                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                       at android.os.Looper.loop(Looper.java:135)
                                                       at android.app.ActivityThread.main(ActivityThread.java:5345)
                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                       at java.lang.reflect.Method.invoke(Method.java:372)
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:947)
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:742)
                                                    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                       at android.content.ContextWrapper.getResources(ContextWrapper.java:86)
                                                       at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:74)
                                                       at android.support.v7.app.AppCompatActivity.getResources(AppCompatActivity.java:555)
                                                       at com.macave.rastreador.AjudaActivity.gerarQuestoes(AjudaActivity.java:42)
                                                       at com.macave.rastreador.AjudaActivity.<init>(AjudaActivity.java:17)
                                                       at java.lang.reflect.Constructor.newInstance(Native Method)
                                                       at java.lang.Class.newInstance(Class.java:1606)
                                                       at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2410) 
                                                       at android.app.ActivityThread.access$800(ActivityThread.java:151) 
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313) 
                                                       at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                       at android.os.Looper.loop(Looper.java:135) 
                                                       at android.app.ActivityThread.main(ActivityThread.java:5345) 
                                                       at java.lang.reflect.Method.invoke(Native Method) 
                                                       at java.lang.reflect.Method.invoke(Method.java:372) 
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:947) 
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:742) 

And this error comes up when I open this Activity:

public class AjudaActivity extends AppCompatActivity {

private List<Questao> listaDeQuestoes = gerarQuestoes();
private ListView lista;
private ArrayAdapter<String> questoes;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.janela_ajuda);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    lista = (ListView) findViewById(R.id.lista);

    final QuestaoAdapter adapter = new QuestaoAdapter(this, listaDeQuestoes);

    lista.setAdapter(adapter);


}



public List<Questao> gerarQuestoes() {


    //As perguntas e as respectivas respostas
    String pergunta1 = this.getResources().getString(R.string.pergunta_1);
    String resposta1 = this.getResources().getString(R.string.resposta_1);

// The error occurs because of the two lines above.

    List<Questao> questoes = new ArrayList<>();
    questoes.add(criaQuestao(pergunta1, resposta1));



    return questoes;
}


public Questao criaQuestao(String pergunta, String resposta) {

    Questao questao = new Questao(pergunta, resposta);

    return questao;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {

   switch(item.getItemId()) {

       case android.R.id.home:
           finish();
           break;

   }

    return super.onOptionsItemSelected(item);
}

}

I need to get these Strings from that file for translation purposes. That's no good.