SQLiteOpenHelper suddenly becomes null

I am new to Android and am making my first app. I made a SQLite table using a contract and helper class. That works fine. I then wrap the database in a ContentProvider, which also seems to work just fine. Here's how I instantiate the SQLiteOpenHelper in my Content Provider class:

public boolean onCreate() {
    mTestDb = new TestDbHelper(getContext());
    return true;
}

I then have two helper methods in the onCreate() method of the MainActivity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    provider = new TestProvider();
    providerInsert();
    providerShow();
}

provider was declared as a class variable previously. Here are the helper methods:

public void providerInsert() {
    ContentValues values = new ContentValues();
    values.put(TestContract.FIRST_COLUMN, 3);
    values.put(TestContract.SECOND_COLUMN, "hello.");
    final Uri uri = getContentResolver().insert(TestContract.CONTENT_URI, values);
}

public void providerShow() {
    Cursor cursor = provider.query(
            TestContract.CONTENT_URI,
            new String[] {TestContract._ID, TestContract.FIRST_COLUMN, TestContract.SECOND_COLUMN},
            null,
            null,
            null
    );
    int id = cursor.getInt(0);
    int first = cursor.getInt(1);
    String second = cursor.getString(2);

    String print = "At column" + id + ": " + first + ", " + second;

    Toast.makeText(getBaseContext(), print, Toast.LENGTH_LONG)
            .show();
}

Heres the kicker: providerInsert() works just fine! I'm getting a values for url and everything looks great. Then, it goes to providerShow() and mTestDb is now suddenly null! Here are the insert() and query() methods for my Content Provider:

public Uri insert(Uri uri, ContentValues values) {
    SQLiteDatabase db = mTestDb.getWritableDatabase();

    switch(matcher.match(uri)) {

        case FIRST_CASE: {
            long id = db.insert(
                    TestContract.TABLE_NAME,
                    null,
                    values
            );

            return ContentUris.withAppendedId(TestContract.CONTENT_URI, id);

        }

        default: {
            return null;
        }
    }
}

public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
    SQLiteDatabase db = mTestDb.getReadableDatabase();
    Cursor rCursor;

    switch(matcher.match(uri)) {

        case FIRST_CASE: {
            rCursor = db.query(
                TestContract.TABLE_NAME,
                projection,
                selection,
                selectionArgs,
                null,
                null,
                sortOrder
            );
            break;
        }

        default: {
            rCursor = null;
        }
    }

    return rCursor;
}

As I previously said, mTestDb becomes null by the time the code gets to query(). I feel like this is just a silly error and it has a simple fix, but I've been pouring over the code for hours to no avail. Could someone please help me out?

Thanks in advance! :)