fix error

app/assets/javascripts/theater.js

@@ -380,6 +380,8 @@ window.GalleryTheater = function() {
                         setMainPic("prev", true);
                     } else if (old_index < index) {
                         setMainPic("next", true);
+                    }else{
+                        setMainPic();
                     }
                     return false;
                 })