center programmatically created window

extension NSWindow {
    public func setFrameOriginToPositionWindowInCenterOfScreen() {
        if let screenSize = screen?.frame.size {
            self.setFrameOrigin(NSPoint(x: (screenSize.width-frame.size.width)/2, y: (screenSize.height-frame.size.height)/2))
        }
    }
}

Objective - C in macOS Catalina , Version 10.15.3

more readable @Wekwa's answer

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    NSWindow * window = NSApplication.sharedApplication.windows[0];
    CGFloat xPos = NSWidth(window.screen.frame)/2 - NSWidth(window.frame)/2;
    CGFloat yPos = NSHeight(window.screen.frame)/2 - NSHeight(window.frame)/2;
    [window setFrame:NSMakeRect(xPos, yPos, NSWidth(window.frame), NSHeight(window.frame)) display:YES];
}

CGFloat xPos = NSWidth([[window screen] frame])/2 - NSWidth([window frame])/2;
CGFloat yPos = NSHeight([[window screen] frame])/2 - NSHeight([window frame])/2;
[window setFrame:NSMakeRect(xPos, yPos, NSWidth([window frame]), NSHeight([window frame])) display:YES];

This puts it at the literal center of the screen, not taking into account the space occupied by the dock and menu bar. If you want to do that, change [[window screen] frame] to [[window screen] visibleFrame].

The question should probably be why [window center] does not work; but assuming that is the case use NSScreen to get the screen coordinates, do the math, and center the window directly.