exception - NSDateにアクセスするためのiOS 6スロー例外

Question

まず、私のアプリケーションは iOS 5 バージョンで問題なく動作します。日付ピッカーからの日付が取得され、コアデータを通過しようとすると例外がスローされますが。ただし、エラーは時々発生します。プログラムの起動時にコンソールに次の警告が表示されます。

2012-10-03 10:58:20.251 MyApp[1115:c07] Application windows are expected to have a root view controller at the end of application launch
2012-10-03 10:58:20.278 MyApp[1115:c07] *** -[__NSCFCalendar components:fromDate:]: date cannot be nil
I mean really, what do you think that operation is supposed to mean with a nil date?
An exception has been avoided for now.
A few of these errors are going to be reported with this complaint, then further violations will simply silently do whatever random thing results from the nil.
Here is the backtrace where this occurred this time (some frames may be missing due to compiler optimizations):
(
    0   CoreFoundation                      0x01d90a75 -[__NSCFCalendar components:fromDate:] + 85
    1   MyApp                               0x000034f8 -[MyApp tableView:cellForRowAtIndexPath:] + 2600
    2   UIKit                               0x00c9df4b -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:withIndexPath:] + 412
    3   UIKit                               0x00c9e01f -[UITableView(UITableViewInternal) _createPreparedCellForGlobalRow:] + 69
    4   UIKit                               0x00c8680b -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow:] + 1863
    5   UIKit                               0x00c9719b -[UITableView layoutSubviews] + 241
    6   UIKit                               0x00c3392d -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 279
    7   libobjc.A.dylib                     0x01bbc6b0 -[NSObject performSelector:withObject:] + 70
    8   QuartzCore                          0x01ac8fc0 -[CALayer layoutSublayers] + 240
    9   QuartzCore                          0x01abd33c _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 468
    10  QuartzCore                          0x01abd150 _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 26
    11  QuartzCore                          0x01a3b0bc _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 324
    12  QuartzCore                          0x01a3c227 _ZN2CA11Transaction6commitEv + 395
    13  QuartzCore                          0x01adeb50 +[CATransaction flush] + 52
    14  UIKit                               0x00be19ff -[UIApplication _reportAppLaunchFinished] + 39
    15  UIKit                               0x00be24e1 -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 2139
    16  UIKit                               0x00bf3315 -[UIApplication handleEvent:withNewEvent:] + 1022
    17  UIKit                               0x00bf424b -[UIApplication sendEvent:] + 85
    18  UIKit                               0x00be5cf8 _UIApplicationHandleEvent + 9874
    19  GraphicsServices                    0x02bd0df9 _PurpleEventCallback + 339
    20  GraphicsServices                    0x02bd0ad0 PurpleEventCallback + 46
    21  CoreFoundation                      0x01cf8bf5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
    22  CoreFoundation                      0x01cf8962 __CFRunLoopDoSource1 + 146
    23  CoreFoundation                      0x01d29bb6 __CFRunLoopRun + 2118
    24  CoreFoundation                      0x01d28f44 CFRunLoopRunSpecific + 276
    25  CoreFoundation                      0x01d28e1b CFRunLoopRunInMode + 123
    26  UIKit                               0x00be17da -[UIApplication _run] + 774
    27  UIKit                               0x00be365c UIApplicationMain + 1211
    28  MyApp                               0x0000257d main + 125
    29  MyApp                                0x000024b5 start + 53
)

これが私のコードです。コアデータに割り当てる前に、NSDate オブジェクトが null かどうかを確認しています。しかし、それでも同じエラーがポップアップします。

StudentDBManager *studentDBManager = [[studentDBManager alloc] init];
    NSManagedObjectContext *context = [studentDBManager getManagedObjectContext];

    Student *student = (Student*)[NSEntityDescription insertNewObjectForEntityForName:@"Student" inManagedObjectContext:context];   
    student.firstName = firstName;
    student.LastName = lastName;
    if(dateOfBirth != NULL)
    {
        NSLog(@"NOT NULLLLLLLLLLLLLLLLLLLlllllLllll");
        NSLog(@"DOB IS :%@", dateOfBirth);
        student.dOB =  dateOfBirth;

    }
    else
    {
        NSLog(@"dateOfBirth IS NULLLLLLLLLLLL");
    }

    student.createdDate = [NSDate date];
    student.title = @"Student";