IOS：pickerView崩溃第11项

Question

我正在从数组填充pickerView。它可以加载到页面中。但是，当您在第11个选项中滚动选择器时，它会因[NSNull rangeOfCharacterFromSet:]: unrecognized selector sent to instance 0x19fc50490错误而崩溃。

我真的不知道下一步该尝试什么，并会对任何建议表示感谢：

这是我的代码：

- (NSInteger)numberOfComponentsInPickerView:
(UIPickerView *)pickerView
{
    return 1;
}

- (NSInteger)pickerView:(UIPickerView *)pickerView
numberOfRowsInComponent:(NSInteger)component{
    return _actnames.count;//THis logs as 30 as there are 30 possibilities
}

- (NSString *)pickerView:(UIPickerView *)pickerView
             titleForRow:(NSInteger)row
            forComponent:(NSInteger)component
{
NSLog(@"before crash");  //last line to display before crash
          return _actnames[row];
    }

-(void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {
    _selectedActName = ([_actnames objectAtIndex:row]);
}

最后一种方法似乎是崩溃的方法。如上所述，当您选择第11个项目时它会崩溃，尽管有30个。

感谢任何想法。

更新

This is method that pulls actnames from core data:

- (id) getActNames{
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Activities"];
    fetchRequest.resultType = NSDictionaryResultType;
    NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"created" ascending:NO];
    [fetchRequest setSortDescriptors:@[sort]];
    NSError *error      = nil;
    self.managedObjectContext = [Model sharedInstance].managedObjectContext;

    NSArray *results    = [self.managedObjectContext executeFetchRequest:fetchRequest
                                                                   error:&error];
    NSMutableArray * actnames = [[results valueForKey:@"actname"] mutableCopy];
    return actnames;
}