AVAssetTrack Combine video not okay in iOS 7 - objective-c

Question

I can successfully combine 2 video into 1 in iOS 6. But I don't know what happen in iOS 7. I got array error like this.
Terminating app due to uncaught exception 'NSRangeException', reason: '* -[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array'
This is part of the code I write. I have shown where error in the code. How should I do?
NSMutableArray *array = [[NSMutableArray alloc]init ];
for(int kk=0;kk < trackRecordingVideoName+1; kk++)
{
[array addObject:[[self userPath] stringByAppendingPathComponent:[NSString stringWithFormat:#"%#%d.mp4",recordingVideoName,kk]]];
NSLog(#"%d is added",kk);
}
videoPathArray= [[NSArray alloc]initWithArray:array];
[videoPathArray retain];
AVMutableComposition *composition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
CMTime startTime = kCMTimeZero;
NSLog(#"videoPathArray.count is %d",videoPathArray.count);
for (NSInteger i=0; i < videoPathArray.count; i++) {
NSLog(#"For loop now is %d and name is %#",i,[videoPathArray objectAtIndex:i]);
NSString *path = (NSString*)[videoPathArray objectAtIndex:i];
NSURL *url = [[NSURL alloc] initFileURLWithPath:path];
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:url options:nil];
[url release];
//*************************************
AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];//This is the error
//*************************************
if(i == 0)
{
[compositionVideoTrack setPreferredTransform:videoTrack.preferredTransform];
}
Boolean ok = [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset duration]) ofTrack:videoTrack atTime:startTime error:nil];
if(ok)
{NSLog(#"can combine in for loop");}
else{NSLog(#"cannot combine in for loop");}
startTime = CMTimeAdd(startTime, [asset duration]);
}

Answer 1

Check the url if its wright, it should point to a real video file, your AVURLAsset don't contain any video media type.