fix bug

ios/Classes/DubbingComposer.swift

@@ -29,10 +29,10 @@ class DubbingComposer {
         let composition = AVMutableComposition()
         
         var videoTimeRange:CMTimeRange?
-        let videoTrack = composition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)
         
         //为合成器添加视频轨道
         if((videoUrl) != nil){
+            let videoTrack = composition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)
             let videoAsset = AVURLAsset(url: videoUrl!, options: nil)
             videoTimeRange = CMTimeRangeMake(start: CMTime.zero, duration: videoAsset.duration)
             do {
@@ -95,14 +95,21 @@ class DubbingComposer {
             assetExport.outputFileType = AVFileType.mp4
             assetExport.outputURL = output
             assetExport.shouldOptimizeForNetworkUse = true
-            assetExport.exportAsynchronously(completionHandler: { 
-                successBlock?()
+            assetExport.exportAsynchronously(completionHandler: {
+                if((assetExport.error) != nil){
+                    print(assetExport.error!)
+                    DispatchQueue.main.async(execute: {
+                        failBlock?("Something wrong on composition")
+                    })
+                }else{
+                    successBlock?()
+                }
                 return
             })
 
         }
         else {
-            DispatchQueue.main.async(execute: { 
+            DispatchQueue.main.async(execute: {
                 failBlock?("Something wrong on composition")
             })
         }