[Core] fix issue with unchecked collect animation option.

AssetStudioUtility/ModelConverter.cs

@@ -92,11 +92,14 @@ namespace AssetStudio
             }
             else
             {
+                if (animationList != null)
+                {
                 foreach (var animationClip in animationList)
                 {
                     animationClipHashSet.Add(animationClip);
                 }
             }
+            }
             ConvertAnimations();
         }