Create a sheet from another class

I can create a leaf using:

[NSApp beginSheet: scorePanel modalForWindow: mainWindow modalDelegate: self didEndSelector: NULL contextInfo: nil];

I can create a sheet when I call this command directly. But when I call the method holding this command from another class, I can not open the error of receiving the sheet:

* Approval error in - [NSApplication _commonBeginModalSessionForWindow: relativeToWindow: modalDelegate: didEndSelector: contextInfo:], / SourceCache / AppKit / AppKit-1038.35 / AppKit.subproj / NSApplication.m: 3100 2011-05-06 18: 52: 43.282 : 903] A modal session requires a modal window