Black video generated after doing video editing in ios

Question

I successfully integrated RosyWriter https://developer.apple.com/library/ios/samplecode/RosyWriter/Introduction/Intro.html#//apple_ref/doc/uid/DTS40011110 in my project and recorded a video from it, In the next phase required a little video editing over the video, using Raywenderlich video editing tutorial at http://www.raywenderlich.com/30200/avfoundation-tutorial-adding-overlays-and-animations-to-videos

the genereated video after editing from the Raywenderlich video editing sample is always black.

I have used both samples separately but the problem is there. Not sure what is the problem with the generated video from RosyWriter. Please Help me