我对Xcode很新。我在YouTube上关注了一个示例，但在启动应用时出现以下错误
2014-05-16 20:45:02.378 CoreData[9335:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Patient''
*** First throw call stack:
(
0 CoreFoundation 0x01b5a1e4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x018d98e5 objc_exception_throw + 44
2 CoreData 0x00238a1b +[NSEntityDescription entityForName:inManagedObjectContext:] + 251
3 CoreData 0x00004235 -[PatientsTableViewController fetechResultsController] + 213
4 CoreData 0x00003bd1 -[PatientsTableViewController viewDidLoad] + 97
5 UIKit 0x006b833d -[UIViewController loadViewIfRequired] + 696
6 UIKit 0x006b85d9 -[UIViewController view] + 35
7 UIKit 0x006e3509 -[UINavigationController rotatingSnapshotViewForWindow:] + 52
8 UIKit 0x00a2ae3c -[UIClientRotationContext initWithClient:toOrientation:duration:andWindow:] + 420
9 UIKit 0x005dfc22 -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:isRotating:] + 1495
10 UIKit 0x005df646 -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:] + 82
11 UIKit 0x005df518 -[UIWindow _setRotatableViewOrientation:updateStatusBar:duration:force:] + 117
12 UIKit 0x005df5a0 -[UIWindow _setRotatableViewOrientation:duration:force:] + 67
13 UIKit 0x005de63a __57-[UIWindow _updateToInterfaceOrientation:duration:force:]_block_invoke + 120
14 UIKit 0x005de59c -[UIWindow _updateToInterfaceOrientation:duration:force:] + 400
15 UIKit 0x005df2f3 -[UIWindow setAutorotates:forceUpdateInterfaceOrientation:] + 870
16 UIKit 0x005e28e6 -[UIWindow setDelegate:] + 449
17 UIKit 0x006bcb77 -[UIViewController _tryBecomeRootViewControllerInWindow:] + 180
18 UIKit 0x005d8474 -[UIWindow addRootViewControllerViewIfPossible] + 591
19 UIKit 0x005d85ef -[UIWindow _setHidden:forced:] + 312
20 UIKit 0x005d886b -[UIWindow _orderFrontWithoutMakingKey] + 49
21 UIKit 0x005e33c8 -[UIWindow makeKeyAndVisible] + 65
22 UIKit 0x00593bc0 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 2097
23 UIKit 0x00598667 -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 824
24 UIKit 0x005acf92 -[UIApplication handleEvent:withNewEvent:] + 3517
25 UIKit 0x005ad555 -[UIApplication sendEvent:] + 85
26 UIKit 0x0059a250 _UIApplicationHandleEvent + 683
27 GraphicsServices 0x03a0df02 _PurpleEventCallback + 776
28 GraphicsServices 0x03a0da0d PurpleEventCallback + 46
29 CoreFoundation 0x01ad5ca5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
30 CoreFoundation 0x01ad59db __CFRunLoopDoSource1 + 523
31 CoreFoundation 0x01b0068c __CFRunLoopRun + 2156
32 CoreFoundation 0x01aff9d3 CFRunLoopRunSpecific + 467
33 CoreFoundation 0x01aff7eb CFRunLoopRunInMode + 123
34 UIKit 0x00597d9c -[UIApplication _run] + 840
35 UIKit 0x00599f9b UIApplicationMain + 1225
36 CoreData 0x00002efd main + 141
37 libdyld.dylib 0x021a1701 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)
这就是我定义函数的方法，但我无法看到它从哪里获取nill？
-(NSFetchedResultsController*)fetechResultsController{
if (_fetechResultsController !=nil) {
return _fetechResultsController;
}
NSFetchRequest *fetchRequest=[[NSFetchRequest alloc] init];
NSManagedObjectContext *context =[self managedObjectContext];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Patient" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]initWithKey:@"patientLastName" ascending:YES];
NSArray *sortdescriptors = [[NSArray alloc]initWithObjects:sortDescriptor, nil];
fetchRequest.sortDescriptors = sortdescriptors;
_fetechResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];
_fetechResultsController.delegate =self;
return _fetechResultsController;
}