fix bug

ios/Classes/DubbingComposer.swift

@@ -71,13 +71,15 @@ class DubbingComposer {
             let currentSec = timeline[i]
             let beginSec = currentSec > preTime ? currentSec - preTime : currentSec
             let beginTime = CMTimeMakeWithSeconds(beginSec, preferredTimescale: 100)
+            if(recordAsset.tracks(withMediaType: AVMediaType.audio).count > 0){
             let recordAssetTrack = recordAsset.tracks(withMediaType: AVMediaType.audio).first!
-            let recordTrack = i % 2 == 0 ? recordTrackA : recordTrackB
-            do {
-                try recordTrack?.insertTimeRange(recordRange, of: recordAssetTrack, at: beginTime)
-            }
-            catch{
-                print("Composer error on record: \(i)")
+                           let recordTrack = i % 2 == 0 ? recordTrackA : recordTrackB
+                           do {
+                               try recordTrack?.insertTimeRange(recordRange, of: recordAssetTrack, at: beginTime)
+                           }
+                           catch{
+                               print("Composer error on record: \(i)")
+                           }
             }
         }