App QUEBRANDO...

Nao sei o que estou fazendo de errado =l

Toda movimentacao que tento fazer no app, mesmo adicionando um simples Texto, o app quebra.

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/Theme.MyApplication">
    <activity android:name=".ActivityMain"
        android:exported="true">
        <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
        </intent-filter>
    </activity>

</application>

public class ActivityMain extends Activity {
 @Override    
 protected void onPostCreate(@Nullable Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    List<String> alunos = new ArrayList<>(Arrays.asList("Igor","Pedro","Joao"));
    //ListView listaDeAlunos = findViewById(R.id.activity_main_listaAlunos);
    //listaDeAlunos.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_expandable_list_item_1,alunos));
    TextView primeiroAluno = findViewById(R.id.textView2);
    primeiroAluno.setText(alunos.get(0));
    setContentView(primeiroAluno);
}
}