Modal Session

Hello, I'm new here.

I would like to open a modal window when I press the mouse button in a view, not when I press a button. How can i do it?

when I do this:

-(IBAction)showMaterialSheet:(id)sender{

[NSApp beginSheet:cargaSheet

  modalForWindow:[inPinta window]

modalDelegate:nil 

  didEndSelector:NULL 

  contextInfo:NULL];

all it's ok.

but when I wont to do this in:

-(void)mouseDown:(NSEvent *)theEvent{

if (pintarRectangulo) {

if (inicio) {

path=[[NSBezierPath alloc] init];

puntoInicial=[theEvent locationInWindow];

[path moveToPoint:[theEvent locationInWindow]];

inicio=!inicio;

                        [NSApp beginSheet:cargaSheet

                   modalForWindow:[inPinta window]

            modalDelegate:nil 

                   didEndSelector:NULL 

              contextInfo:NULL];

                 }

[self setNeedsDisplay:YES];

}

I have this error:

2009-12-24 07:17:56.287 Secciones[2484:a0f] *** Assertion failure in -[NSApplication _commonBeginModalSessionForWindow:relativeToWindow:modalDelegate:didEndSelector:contextInfo:], /SourceCache/AppKit/AppKit-1038.25/AppKit.subproj/NSApplication.m:3100

2009-12-24 07:17:56.288 Secciones[2484:a0f] Modal session requires modal window

thanks, 

Joaquín