Cocoa: Simultaneous fullscreen toggle & opening of modal dialog fails

#import "AppDelegate.h"

@interface AppDelegate ()

@property (strong) NSWindow *window;
@property (strong) NSButton *fullscreenButton;
@property (strong) NSButton *saveOnlyButton;

@end

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    // Create the window
    NSRect frame = NSMakeRect(0, 0, 600, 400);
    self.window = [[NSWindow alloc] initWithContentRect:frame
                                               styleMask:(NSWindowStyleMaskTitled | NSWindowStyleMaskClosable | NSWindowStyleMaskResizable)
                                                 backing:NSBackingStoreBuffered
                                                   defer:NO];
    [self.window setTitle:@"Fullscreen & Save Panel"];
    [self.window makeKeyAndOrderFront:nil];
    
    // Create the fullscreen and save button
    self.fullscreenButton = [[NSButton alloc] initWithFrame:NSMakeRect(200, 180, 200, 50)];
    [self.fullscreenButton setTitle:@"Go Fullscreen & Save"];
    [self.fullscreenButton setTarget:self];
    [self.fullscreenButton setAction:@selector(fullscreenButtonClicked:)];
    [[self.window contentView] addSubview:self.fullscreenButton];
    
    // Create the save only button
    self.saveOnlyButton = [[NSButton alloc] initWithFrame:NSMakeRect(200, 100, 200, 50)];
    [self.saveOnlyButton setTitle:@"Open Save Panel"];
    [self.saveOnlyButton setTarget:self];
    [self.saveOnlyButton setAction:@selector(saveOnlyButtonClicked:)];
    [[self.window contentView] addSubview:self.saveOnlyButton];
    
    [self.window center];
}

- (void)applicationWillTerminate:(NSNotification *)aNotification {
    // Insert code here to tear down your application
}

- (void)fullscreenButtonClicked:(id)sender {
    // Go fullscreen
    [self.window toggleFullScreen:nil];
    
    // Open "Save As" panel (modal)
    [self openSavePanel];
}

- (void)saveOnlyButtonClicked:(id)sender {
    // Open "Save As" panel (modal)
    [self openSavePanel];
}

- (void)openSavePanel {
    // Open "Save As" panel in modal mode
    NSSavePanel *savePanel = [NSSavePanel savePanel];
    [savePanel setTitle:@"Save As"];
    [savePanel setMessage:@"Choose a location to save the file."];
    [savePanel setAllowedFileTypes:@[@"txt", @"pdf"]];
    
    // Make the save panel modal
    NSInteger result = [savePanel runModal];  // This will make the panel modal
    
    if (result == NSModalResponseOK) {
        NSURL *selectedFileURL = [savePanel URL];
        // Handle the file URL here if necessary
        NSLog(@"File will be saved at: %@", selectedFileURL.path);
    }
}

@end