QColorDialog will only open native Mac OS dialog once, then reverts to non-native version

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QMainWindow>

class QColorDialog;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();

private Q_SLOTS:
    void openColor();
    void colorDialogFinished(int result);

private:
    QColorDialog *color_dialog;
};

#endif // MAINWINDOW_H

#include "MainWindow.h"
#include <QPushButton>
#include <QVBoxLayout>
#include <QColorDialog>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget *widget = new QWidget();
    QVBoxLayout *layout = new QVBoxLayout(widget);
    QPushButton *button = new QPushButton("Push Me");
    layout->addWidget(button);
    setCentralWidget(widget);
    connect(button, SIGNAL(clicked()), this, SLOT(openColor()));
}

MainWindow::~MainWindow()
{

}

void MainWindow::openColor()
{
    color_dialog = new QColorDialog();
    color_dialog->setOption(QColorDialog::NoButtons, true);
    connect(color_dialog, SIGNAL(finished(int)), this, SLOT(colorDialogFinished(int)));
    color_dialog->show();
}

void MainWindow::colorDialogFinished(int result)
{
    color_dialog->deleteLater();
}