Print your array count. It must be an empty array that's why your app is getting crashed. Check that.
NSlog(@"array count => %d",[videoAsset tracksWithMediaType:AVMediaTypeVideo]);
and replace your code by below code to prevent crashes.
AVMutableCompositionTrack *compositionVideoTrack =
[mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
NSArray *dataSourceArray = [NSArray arrayWithArray: [videoAsset tracksWithMediaType:AVMediaTypeVideo];
NSlog(@"array count => %d",[dataSourceArray count]);
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
ofTrack:([dataSourceArray count]>0)?[dataSourceArray objectAtIndex:0]:nil
atTime:kCMTimeZero
error:nil];