Android implement CursorLoader to SQLite database loader

I want to have two ListView in one Activity. I am trying to use Fragments. Data will loaded to ListViews from Sqlite database. I implement AsyncTaskLoader for working with SQllite used source code of cursorloader, but method doInBackground() issue mistake. This is my code:

public class DatabaseLoader extends AsyncTaskLoader<Cursor> {

final ForceLoadContentObserver mObserver;

SQLiteDatabase mDb;
String mTable;
String[] mColumns;
String mSelection;
String[] mSelectionArgs;
String mSortOrder;
Cursor mCursor;

/* Runs on a worker thread */
@Override
public Cursor loadInBackground() {
     Cursor cursor = mDb.query(mTable, mColumns, mSelection,
            mSelectionArgs, null, null, mSortOrder, null);

   if (cursor != null) {

       cursor.getCount();
       registerContentObserver(cursor, mObserver);
    }
    return cursor;
}


void registerContentObserver(Cursor cursor, ContentObserver observer) {
    cursor.registerContentObserver(mObserver);
}

/* Runs on the UI thread */
@Override
public void deliverResult(Cursor cursor) {
    if (isReset()) {
        // An async query came in while the loader is stopped
        if (cursor != null) {
            cursor.close();
        }
        return;
    }
    Cursor oldCursor = mCursor;
    mCursor = cursor;

    if (isStarted()) {
        super.deliverResult(cursor);
    }

    if (oldCursor != null && oldCursor != cursor && !oldCursor.isClosed()) {
        oldCursor.close();
    }
}


public DatabaseLoader(Context context) {
    super(context);
    mObserver = new ForceLoadContentObserver();
}


public DatabaseLoader(Context context, String table, String[] columns, String selection,
        String[] selectionArgs, String sortOrder) {
    super(context);
    mObserver = new ForceLoadContentObserver();
    mTable  = table;
    mColumns = columns;
    mSelection = selection;
    mSelectionArgs = selectionArgs;
    mSortOrder = sortOrder;
}

@Override
protected void onStartLoading() {
    if (mCursor != null) {
        deliverResult(mCursor);
    }
    if (takeContentChanged() || mCursor == null) {
        forceLoad();
    }
}

/**
 * Must be called from the UI thread
 */
@Override
protected void onStopLoading() {
    // Attempt to cancel the current load task if possible.
    cancelLoad();
}

@Override
public void onCanceled(Cursor cursor) {
    if (cursor != null && !cursor.isClosed()) {
        cursor.close();
    }
}

@Override
protected void onReset() {
    super.onReset();

    // Ensure the loader is stopped
    onStopLoading();

    if (mCursor != null && !mCursor.isClosed()) {
        mCursor.close();
    }
    mCursor = null;
}

public String getTable() {
    return mTable;
}

public void setTable(String table) {
    mTable = table;
}

public String[] getColumns() {
    return mColumns;
}

public void setColumns(String[] columns) {
    mColumns = columns;
}

public String getSelection() {
    return mSelection;
}

public void setSelection(String selection) {
    mSelection = selection;
}

public String[] getSelectionArgs() {
    return mSelectionArgs;
}

public void setSelectionArgs(String[] selectionArgs) {
    mSelectionArgs = selectionArgs;
}

public String getSortOrder() {
    return mSortOrder;
}

public void setSortOrder(String sortOrder) {
    mSortOrder = sortOrder;
}

@Override
public void dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args) {
    super.dump(prefix, fd, writer, args);
//  writer.print(prefix); writer.print("mUri="); writer.println(mUri);
    writer.print(prefix); writer.print("mTable="); writer.println(mTable);
    writer.print(prefix); writer.print("mColumns="); writer.println(Arrays.toString(mColumns));
    writer.print(prefix); writer.print("mSelection="); writer.println(mSelection);
    writer.print(prefix); writer.print("mSelectionArgs="); writer.println(Arrays.toString(mSelectionArgs));
    writer.print(prefix); writer.print("mSortOrder="); writer.println(mSortOrder);
    writer.print(prefix); writer.print("mCursor="); writer.println(mCursor);
//  writer.print(prefix); writer.print("mContentChanged=");
    writer.println(mContentChanged);
    }
}

Can help me somebody. Why method doInBackground() don't work?