fix yjs#645 yjs/y-utility#8

Author: dmonad

README.md

@@ -148,7 +148,6 @@ are implemented in separate modules.
 | [Slate](https://github.com/ianstormtaylor/slate) | ✔ | [slate-yjs](https://github.com/bitphinix/slate-yjs) | [demo](https://bitphinix.github.io/slate-yjs-example) |
 | [BlockSuite](https://github.com/toeverything/blocksuite) | ✔ | (native) | [demo](https://blocksuite-toeverything.vercel.app/?init) |
 | [Lexical](https://lexical.dev/) | ✔ | (native) | [demo](https://lexical.dev/docs/collaboration/react#see-it-in-action) |
-
 | [valtio](https://github.com/pmndrs/valtio) |  | [valtio-yjs](https://github.com/dai-shi/valtio-yjs) | [demo](https://codesandbox.io/s/valtio-yjs-demo-ox3iy) |
 | [immer](https://github.com/immerjs/immer) |  | [immer-yjs](https://github.com/sep2/immer-yjs) | [demo](https://codesandbox.io/s/immer-yjs-demo-6e0znb) |
 | React / Vue / Svelte / MobX | | [SyncedStore](https://syncedstore.org) | [demo](https://syncedstore.org/docs/react) |

src/utils/UndoManager.js

@@ -118,12 +118,13 @@ const popStackItem = (undoManager, stack, eventType) => {
     })
     _tr = transaction
   }, undoManager)
-  if (undoManager.currStackItem != null) {
+  const res = undoManager.currStackItem
+  if (res != null) {
     const changedParentTypes = _tr.changedParentTypes
-    undoManager.emit('stack-item-popped', [{ stackItem: undoManager.currStackItem, type: eventType, changedParentTypes, origin: undoManager }, undoManager])
+    undoManager.emit('stack-item-popped', [{ stackItem: res, type: eventType, changedParentTypes, origin: undoManager }, undoManager])
     undoManager.currStackItem = null
   }
-  return undoManager.currStackItem
+  return res
 }
 
 /**