Problema na aula 3 - usando Fragment

Boa tarde, estou com um problema ao executar o projeto, segui os passos da aula, porem não esta aparecendo a lista de provas como deveria aparecer conforme a vidro aula. segue print e codigos.: https://postimg.org/image/vngg6tt8x/

ListaProvasFragment:

public class ListaProvasFragment extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_lista_provas, container, false);

        List<String> topicosPort = Arrays.asList("Sujeito", "Objeto direto", "Objeto indireto");
        Prova provaPortugues = new Prova("Portugues", "25/05/2016", topicosPort);

        List<String> topicosMat = Arrays.asList("Equacoes de segundo grau", "Trigonometria");
        Prova provaMatematica = new Prova("Matematica", "27/05/2016", topicosMat);

        List<Prova> provas = Arrays.asList(provaPortugues, provaMatematica);

        ArrayAdapter<Prova> adapter = new ArrayAdapter<>(getContext(), android.R.layout.simple_list_item_1, provas);

        ListView lista = (ListView) view.findViewById(R.id.provas_lista);
        lista.setAdapter(adapter);


        lista.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Prova prova = (Prova) parent.getItemAtPosition(position);
                Toast.makeText(getContext(), "Clicou na Prova: " + prova, Toast.LENGTH_SHORT).show();

                Intent vaiParaDetalhes = new Intent(getContext(), DetalhesProvasAcitivity.class);
                vaiParaDetalhes.putExtra("prova", prova);
                startActivity(vaiParaDetalhes);
            }
        });
        return view;
    }
}

ProvasActivity:

public class ProvasActivity extends AppCompatActivity {

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

        FragmentManager fragmentManager =  getSupportFragmentManager();
        FragmentTransaction tx = fragmentManager.beginTransaction();
        tx.replace(R.id.frame_principal,new ListaProvasFragment());
        tx.commit();

    }
}

ProvasTabletActivity:

public class ProvasTabletActivity extends AppCompatActivity {

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

activity_provas.xml

<FrameLayout
    android:id="@+id/frame_principal"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

</FrameLayout>

activity_provas_tablet.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:orientation="horizontal">

    <include layout="@layout/activity_provas"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight = "1"
        />

    <include layout="@layout/activity_detalhes_provas_acitivity"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        />

</LinearLayout>

fragment_lista_provas.xml:

<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:cardElevation="5dp"
    app:cardUseCompatPadding="true">

    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/provas_lista"/>



</android.support.v7.widget.CardView>