Problems with QPainter

[franco.amato]

Hi to all,
I'm writing a config dialog that I exec from my main application.
With the dialog the user can choose the colors that will be used to draw an audio waveform.
In the dialog I would show a preview of the waveform so I added a little QFrame named previewFrame and in the paintEvent I would
draw a little sine function over it.

This is part of the code of the paint event.

void ConfigDlg::paintEvent( QPaintEvent* event )
{
	int h = ui.previewFrame->height();
	int w = ui.previewFrame->width();
 
        QPainter p( ui.previewFrame ); //<--give error!!!!!!!!
	p.setRenderHint( QPainter::Antialiasing, true );
	p.fillRect( 0, 0, w, h, bgColor );
	QPen pen = QPen( Qt::darkRed, 1 );
}

void ConfigDlg::paintEvent( QPaintEvent* event )
{
	int h = ui.previewFrame->height();
	int w = ui.previewFrame->width();

        QPainter p( ui.previewFrame ); //<--give error!!!!!!!!
	p.setRenderHint( QPainter::Antialiasing, true );
	p.fillRect( 0, 0, w, h, bgColor );
	QPen pen = QPen( Qt::darkRed, 1 );
}

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The problem is that I get the following error

QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setRenderHint: Painter must be active to set rendering hints

Where the code is wrong?

Best Regards