XMPPFramework, NSManagedObjectContext is nil

XMPPFramework, NSManagedObjectContext is nil

I'm working with XMPPFramework on iOS. When I'm trying to fetch the data
from XMPPCoreDataStorage
XMPPMessageArchivingCoreDataStorage *storage =
[XMPPMessageArchivingCoreDataStorage sharedInstance];
NSManagedObjectContext *moc = [storage mainThreadManagedObjectContext];
NSEntityDescription *entityDescription = [NSEntityDescription
entityForName:@"XMPPMessageArchiving_Message_CoreDataObject"
inManagedObjectContext:moc];
NSFetchRequest *request = [[NSFetchRequest alloc]init];
[request setEntity:entityDescription];
NSError *error;
NSArray *messages = [moc executeFetchRequest:request error:&error];
the managed object context is nill
NSManagedObjectContext *moc = [storage mainThreadManagedObjectContext];
Any ideas?