calling QMessageBox from a worker thread

[prkhr4u]

I am emitting a signal from a worker thread and connecting it to a slot in GUI thread of MainWindow.
In the slot I am invoking a QMessageBox and displaying some error message.
Now I want the worker thread to stop working until I click OK on the QMessageBox.
Here is how I am doing it:

//worker thread

emit CameraDisconnectSignal("no camera present");

emit CameraDisconnectSignal("no camera present");

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//GUI thread

void MainWindow::CameraDisconnectSlot(QString q)
{
           QMessageBox messageBox;
           messageBox.critical(0,"Error",q);
           messageBox.setFixedSize(500,200);
}

void MainWindow::CameraDisconnectSlot(QString q)
{
           QMessageBox messageBox;
           messageBox.critical(0,"Error",q);
           messageBox.setFixedSize(500,200);
}

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First of all I am not able to get any MessageBox and secondly I want to stop worker thread till the user clicks OK on GUI thread.
How can I acheive this??