Modal sheet window not centered

I'm trying to open a sheet window on my cocoa app:

MyWindowController *controller = [[MyWindowController alloc] init];

NSWindow *window = [NSApp mainWindow];

[NSApp beginSheet:controller.window modalForWindow:window modalDelegate:self didEndSelector:nil contextInfo:nil];

The problem is that the sheet is not attached to the current window, instead it's shown in the bottom left of the screen. I've tried开发者_开发知识库 to use self.view window but nothing changed. What should I do to have the sheet positioned at the top-enter of my window?

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Make sure that Visible At Launch is unchecked for your sheet window.