title: PHPhotoLibrary crashed when getting placeholderForCreatedAsset
body: <p>I try to put image to custom album with this code:</p>
  
  <pre><code>PHAssetCollection *album = [self getMyAlbum];
  UIImage *image = [self getMyImage];
  
  [[PHPhotoLibrary sharedPhotoLibrary] performChanges:^{
      PHAssetChangeRequest *createAssetRequest = [PHAssetChangeRequest creationRequestForAssetFromImage:image];
  
      PHObjectPlaceholder * placeHolder = createAssetRequest.placeholderForCreatedAsset;
  
      PHAssetCollectionChangeRequest *albumChangeRequest = [PHAssetCollectionChangeRequest changeRequestForAssetCollection:album];
  
      if(placeHolder){
          [albumChangeRequest addAssets:@[ placeHolder ]];
      }
  
  } completionHandler:^(BOOL success, NSError *error) {
      //doesen't matter
  }];
  </code></pre>
  
  <p>So, i get many errors in users logs in this row <code>createAssetRequest.placeholderForCreatedAsset</code></p>
  
  <p>like</p>
  
  <blockquote>
    <p>1 CoreFoundation __exceptionPreprocess + 1245624</p>
    
    <p>2 libobjc.A.dylib objc_exception_throw + 34136</p>
    
    <p>3 Photos __48-[PHChangeRequestHelper generateUUIDIfNecessary]_block_invoke + 116552</p>
    
    <p>2 libdispatch.dylib _dispatch_semaphore_wait_slow + 79828</p>
    
    <p>3 Photos -[PHChangeRequestHelper generateUUIDIfNecessary] + 115992</p>
    
    <p>4 Photos -[PHAssetCreationRequest placeholderForCreatedAsset] + 244020</p>
  </blockquote>
  
  <p>so <code>[PHChangeRequestHelper generateUUIDIfNecessary]</code> give me crash.</p>
  
  <p>I see this only on <strong>iOS > 10</strong>, and I can't reproduct this in the simulator.</p>
  
  <p>What is it? How to fix that?</p>