macOS unrecognized selector sent to instance

#import <Cocoa/Cocoa.h>


@interface AppController : NSObject {
    IBOutlet NSTextField *tagTitle;
    IBOutlet NSTableView *filesTableView;
    NSArray              *openedFiles;
}
- (IBAction)showOpenPanel:(id)sender;
- (void)openPanelDidEnd:(NSOpenPanel*)sheet
             returnCode:(int)returnCode
            contextInfo:(void*)contextInfo;
@end

#import "AppController.h"


@implementation AppController

- (IBAction)showOpenPanel:(id)sender
{
    NSOpenPanel *panel = [NSOpenPanel openPanel];

    [panel setCanChooseFiles: YES];
    [panel setCanChooseDirectories: NO];
    [panel setAllowsMultipleSelection: YES];
    NSArray *fTypes = [NSArray arrayWithObjects: @"txt", nil];

    [panel beginSheetForDirectory: nil
                             file: nil
                            types: fTypes
                   modalForWindow: [filesTableView window]
                    modalDelegate: self
                   didEndSelector: @selector(openPanelDidEnd:
                                            returnCode:
                                            contextInfo:)
                      contextInfo: nil];
}

- (void)openPanelDidEnd:(NSOpenPanel*)sheet
             returnCode:(int)returnCode
            contextInfo:(void*)contextInfo
{
    if (returnCode == NSOKButton) {
        openedFiles = [sheet filenames];
        [filesTableView reloadData];
    }
    // NSRunAlertPanel(@"Test: ", @"Test", nil, nil, nil);
}

- (int)numberOfRowsInTableView:(NSTableView *)tv
{
    return [openedFiles count];
}

- (id)tableView:(NSTableView *)tv
    objectValueForTableColumn:(NSTableColumn *)tableColumn
                          row:(int)row
{
    NSString *filename = [openedFiles objectAtIndex: 0];
    return filename;
}

@end

NSRunAlertPanel(@"Test: ", @"Test", nil, nil, nil);