fixed exporting sprites

HISTORY.md

@@ -32,6 +32,7 @@
 ### 2021-07-23
 * byob, objects, gui, store: support custom categories in libraries
 * gui, byob: arrange custom categories alphabetically
+* store: fixed exporting sprites
 
 ### 2021-07-22
 * store: serialize user defined block palettes

src/store.js

@@ -1816,7 +1816,7 @@ StageMorph.prototype.toXML = function (serializer) {
 };
 
 SpriteMorph.prototype.toXML = function (serializer) {
-    var idx = serializer.root.sprites.asArray().indexOf(this) + 1,
+    var idx = serializer.scene.sprites.asArray().indexOf(this) + 1,
         costumeIdx = this.getCostumeIdx(),
         noCostumes = this.inheritsAttribute('costumes'),
         noSounds = this.inheritsAttribute('sounds'),