only use frames that are ready.

pandora/item/models.py

@@ -1380,6 +1380,7 @@ class Item(models.Model):
             offset = 0
             for f in self.files.filter(selected=True, is_video=True).order_by('sort_path'):
                 for ff in f.frames.all().order_by('position'):
+                    if ff.frame:
                         frames.append({
                             'position': offset + ff.position,
                             'path': ff.frame.path,