应用程序试图在目标上推送一个nil视图控制器

Question

使用tableview并尝试打开 collectionviewcontroller，其中包含导航控制器的故事板和根视图控制器，当按下tableview单元格时。但是收到错误消息

应用程序试图在目标上推送一个nil视图控制器

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
UIViewController *viewController;
switch (indexPath.row) {
    case PDF: 
        viewController = [[[PDFExampleViewController alloc] init]autorelease];

        break;
    case IMAGE: 
//Rootviewcontroller of the navigation controller have no identifier. showDetail is the segue of the rootviewcontroller.

 viewController = [self.storyboard instantiateViewControllerWithIdentifier:@"showDetail"];
        break;
    case PROCEDURAL:
        viewController = [[[ProceduralExampleViewController alloc] init] autorelease]; 
        break;
    default: 
        viewController = [[[UIViewController alloc] init] autorelease];
} 


[self.navigationController pushViewController:viewController animated:YES];
}

感谢您的帮助。