title: UICollectionView cellForItemAtIndexPath is nil
body: <p>I have a function that updates existing UICollectionView. 
  The UICollectionView is created, I can see it, but when I want to access its cells to update it, they are nil.</p>
  
  <pre><code>-(void)finishExam{
  
      for (int i = 0; i &lt; [self.questionsOverviewCollection numberOfItemsInSection:0]; i++) {
  
          NSLog(@"self.questionsOverviewCollection - %@",self.questionsOverviewCollection);
          NSLog(@"cell - %@",[self.questionsOverviewCollection cellForItemAtIndexPath:[NSIndexPath indexPathForRow:i inSection:0]]);
          NSLog(@"overviewCell - %@",(OverviewCell*)[self.questionsOverviewCollection cellForItemAtIndexPath:[NSIndexPath indexPathForRow:i inSection:0]]);
          NSLog(@"numOfCells - %d", [self.questionsOverviewCollection numberOfItemsInSection:0]);
  
          OverviewCell *cell = (OverviewCell*)[self.questionsOverviewCollection cellForItemAtIndexPath:[NSIndexPath indexPathForRow:i inSection:0]];
              [cell finishExam];
      }
  }
  
  
  - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
  
      OverviewCell *cell = (OverviewCell*)[collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
  
      [cell someSetUp];
  
      return cell;
  }
  </code></pre>
  
  <p>Log:</p>
  
  <pre><code>self.questionsOverviewCollection - &lt;UICollectionView: 0xa1abc00; frame = (14 219; 217 441); clipsToBounds = YES; opaque = NO; autoresize = W+H; gestureRecognizers = &lt;NSArray: 0xe0617a0&gt;; layer = &lt;CALayer: 0xe0bbb00&gt;; contentOffset: {0, 0}&gt; collection view layout: &lt;UICollectionViewFlowLayout: 0xe0cc3f0&gt;
  cell - (null)
  overviewCell - (null)
  numOfCells - 30
  </code></pre>