由于异常导致代码中的新错误

Question

我最近在我的应用程序中发布了一个关于错误的问题，但我现在设法用这部分代码修复了这个错误，但我现在得到了同样的错误（主题1：信号SIGABRT ）但是对于代码的不同部分。我已经包含了头文件和实现文件，以及main.m文件。我通过在//之后写下错误来指示错误正在使用哪一行。调试器发出应用程序崩溃原因的消息是：

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Shift'

我认为这是指我在错误所在的代码行中的@“Shift”，但我不知道为什么它不合法。请你能帮我找出原因。

NameSaveAsViewController.h

#import <UIKit/UIKit.h>
#import "Foundation/Foundation.h"

@interface NameSaveAsViewController : UIViewController {
}


@property (nonatomic, weak) IBOutlet UITextField *saveAsText;
@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator 
*persistantStoreCoordinator;


- (IBAction)saveAs:(id)sender;
- (IBAction)textFieldDidBeginEditing:(UITextField *)textField;
- (IBAction)textFieldDidEndEditing:(UITextField *)textField;
- (IBAction)dismissKeyboard:(id)sender;

@end

NameSaveAsViewController.m

#import "NameSaveAsViewController.h"
#import "AppDelegate.h"
#import "Shift.h"

@interface NameSaveAsViewController ()

@end

@implementation NameSaveAsViewController
@synthesize saveAsText;

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistantStoreCoordinator = _persistantStoreCoordinator;


- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {

    }
    return self; 
} 


-(void)viewDidLoad {

[super viewDidLoad];

    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    _managedObjectContext = [appDelegate managedObjectContext];

    UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(dismissKeyboard:)];
    [self.view addGestureRecognizer:tap];
    saveAsText.placeholder = @"Shift Name";

}

- (void)dismissKeyboard:(id)sender
{
    if ([saveAsText isEditing]) {
        [saveAsText resignFirstResponder];
    }
}

-(void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

- (void)textFieldDidBeginEditing:(UITextField *)textField {


}

- (void)textFieldDidEndEditing:(UITextField *)textField {

}

- (void)saveAs:(id)sender
{
    Shift *shift = [NSEntityDescription insertNewObjectForEntityForName:@"Shift" inManagedObjectContext:_managedObjectContext]; //this is the line that xcode tells  me has the error
    [shift setMake:saveAsText.text];

    NSError *error = nil;
    if (![_managedObjectContext save:&error]) {

    }
    saveAsText.text = NULL;
}

@end