Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: setObjectForKey: key cannot be nil'

When i call this method code is crashing and throw the error like "setObjectForKey: key cannot be nil"

Please guide me....

Thanks! Here is my code:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{

    NSArray* groupKeyName = [self.groupsDict allKeysForObject:collectionView];
    NSArray* groupList = self.groupedServiceInfoDict[groupKeyName[0]];
    NSDictionary* dict = groupList[indexPath.item];


    static NSString *CellIdentifier = @"GroupCell";

    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
    cell.layer.borderWidth = 1.0f;

    cell.layer.borderColor = [UIColor colorWithRed:225/255.0f green:225/255.0f blue:225/255.0f alpha:1.0f].CGColor;

    OnGoServiceCategories *serviceCategory = [self.groupedServiceInfoDict[@"grouplist"] objectAtIndex:indexPath.item];

    UILabel* nameLabel = (UILabel*)[cell viewWithTag:2];
    nameLabel.text = serviceCategory.Name;

    NSString* imageUrl = [dict objectForKey:@"Image_URL"];


        UIImage *image = [[CXResourceManager sharedInstance] imageForPath:imageUrl];
        UIImageView* imageView = (UIImageView*)[cell viewWithTag:4];

        if(!image)
        {
         [[OnGoDownloadManager sharedDownloadManager] downloadDataWithURLString:imageUrl
                                                                          dataType:DATA_TYPE_BINARY finishBlock:^(OnGoDownloadData *downloadData){
                                                                              if (downloadData.downloadStatus == DOWNLOAD_STATUS_SUCCESS)
                                                                              {
                                                                                  UIImage *image = [UIImage imageWithData:downloadData.data];
                                                                                      imageView.contentMode = UIViewContentModeScaleAspectFit;
                                                                                      [imageView setImage:image];

                                                                                      [[CXResourceManager sharedInstance] setImage:image forPath:imageUrl];

                                                                                      CGSize size = [image size];

                                                                                      CGFloat deltaWidth = size.width - imageView.frame.size.width;

                                                                                      CGFloat deltaHeight = size.height - imageView.frame.size.height;

                                                                                      NSValue* value = [self.sizesDict objectForKey:[NSString stringWithFormat:@"%ld", (long)indexPath.item]];

                                                                                      CGSize defaultSize =  [value CGSizeValue];

                                                                                      CGSize newSize;
                                                                                      newSize.height = defaultSize.height + deltaHeight;
                                                                                      newSize.width = defaultSize.width ;

                                                                                      [self.sizesDict setObject:[NSValue valueWithCGSize:newSize] forKey:[NSString stringWithFormat:@"%ld", (long)indexPath.item]];

                                                                                      [self.collectionView reloadItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
                                                                                   }
                                                                          }];
        }

        else
        {
            [imageView setImage:image];
        }

    return cell;

}