Why AVAssetImageGenerator generateCGImagesAsynchronouslyForTimes crashes the app - ios

Question

I am trying to extract 2 frames per second from a video using generateCGImagesAsynchronouslyForTimes. But my app crashes. I am monitoring the memory usage but its not going any way up than 14 mb.
Here is the code:
- (void) createImagesFromVideoURL:(NSURL *) videoUrl atFPS: (int) reqiuredFPS completionBlock: (void(^) (NSMutableArray *frames, CGSize frameSize)) block
{
NSMutableArray *requiredFrames = [[NSMutableArray alloc] init];
AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoUrl options:nil];
AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
generator.requestedTimeToleranceAfter = kCMTimeZero;
generator.requestedTimeToleranceBefore = kCMTimeZero;
generator.appliesPreferredTrackTransform = YES;
UIImage *sampleGeneratedImage;
for (Float64 i = 0; i < CMTimeGetSeconds(asset.duration) * reqiuredFPS ; i++)
{
CMTime time = CMTimeMake(i, reqiuredFPS);
NSError *err;
CMTime actualTime;
CGImageRef image = [generator copyCGImageAtTime:time actualTime:&actualTime error:&err];
if (! err)
{
sampleGeneratedImage = [[UIImage alloc] initWithCGImage:image];
break;
}
}
//Get the maximum size from 1 frame
generator.maximumSize = [self getMultipleOf16AspectRatioForCurrentFrameSize:sampleGeneratedImage.size];
NSMutableArray *requestedFrameTimes = [[NSMutableArray alloc] init];
for (Float64 i = 0; i < CMTimeGetSeconds(asset.duration) * reqiuredFPS ; i++)
{
CMTime time = CMTimeMake(i, reqiuredFPS);
[requestedFrameTimes addObject:[NSValue valueWithCMTime:time]];
}
[generator generateCGImagesAsynchronouslyForTimes:[requestedFrameTimes copy] completionHandler:^(CMTime requestedTime, CGImageRef _Nullable image, CMTime actualTime, AVAssetImageGeneratorResult result, NSError * _Nullable error) {
if (image)
{
UIImage *generatedImage = [UIImage imageWithCGImage:image];
[requiredFrames addObject:generatedImage];
}
if (CMTimeCompare(requestedTime, [[requestedFrameTimes lastObject] CMTimeValue]) == 0)
{
NSLog(#"Image processing complete");
dispatch_async(dispatch_get_main_queue(), ^{
block(requiredFrames, generator.maximumSize);
});
}
else
{
NSLog(#"Getting frame at %lld", actualTime.value/actualTime.timescale);
}
}];
}