NullPointerException on Android [duplicate]

This question already has an answer here:

• What is a Null Pointer Exception, and how do I fix it? 12 answers

I have a very simple app that I'm developing in Android Studio

I'm getting a NullPointerException and I don't know why. Maybe a new set of fresh eyes could pount me in the correct direction.

I'm trying to populate a listview on right after start. the data is loaded from sqlite

so, the in the mainactivity.java I have:

DatabaseHelper myDb;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);


           Cursor cursor = myDb.getAllviaturas();
    String[] fromFieldNames = new String[] {DatabaseHelper.KEY_MATRICULA, DatabaseHelper.KEY_NOME};
    int[] toViewIDs = new int[] {R.id.textViewMatricula,R.id.textViewNome};
    SimpleCursorAdapter myCursorAdapter;
    myCursorAdapter = new SimpleCursorAdapter(getBaseContext(),R.layout.listview_each_item,cursor,fromFieldNames,toViewIDs,0);
    ListView myList = (ListView) findViewById(R.id.lvViaturas);
    myList.setAdapter(myCursorAdapter);

and in the DatabseHelper.java

public Cursor getAllviaturas() {
    SQLiteDatabase db = this.getReadableDatabase();
    String where = null;
    Cursor c =  db.query(true, TABLE_VIATURAS, ALL_VIATURAS_KEYS, where, null, null, null, null, null);
    if (c != null) {
        c.moveToFirst();
    }
    return c;

Any Help?