alexeyinkin opened a new issue, #21950:
URL: https://github.com/apache/beam/issues/21950
### What happened?
On a development machine, the exception is thrown after startup. Sometimes
more than one.
```
errors.dart:251 Uncaught (in promise) Error: A PlaygroundState was used
after being disposed.
Once you have called dispose() on a PlaygroundState, it can no longer be
used.
at Object.throw_ [as throw] (errors.dart:251:49)
at change_notifier.dart:125:9
at [_debugAssertNotDisposed] (change_notifier.dart:130:14)
at playground_state.PlaygroundState.new.notifyListeners
(change_notifier.dart:304:12)
at playground_state.PlaygroundState.new.setOutputResult
(playground_state.dart:125:5)
at playground_state.PlaygroundState.new.setExample
(playground_state.dart:105:5)
at playground_page_providers.dart:77:38
at _RootZone.runUnary (zone.dart:1685:54)
at _FutureListener.then.handleValue (future_impl.dart:147:18)
at handleValueCallback (future_impl.dart:766:44)
at _Future._propagateToListeners (future_impl.dart:795:13)
at [_completeWithValue] (future_impl.dart:566:5)
at future.dart:527:22
at _RootZone.runUnary (zone.dart:1685:54)
at _FutureListener.then.handleValue (future_impl.dart:147:18)
at handleValueCallback (future_impl.dart:766:44)
at _Future._propagateToListeners (future_impl.dart:795:13)
at [_complete] (future_impl.dart:557:7)
at stream.dart:1035:13
at _RootZone.runGuarded (zone.dart:1606:10)
at sendDone (stream_impl.dart:394:13)
at _RootZone.run (zone.dart:1679:54)
at _FutureListener.whenComplete.handleWhenComplete
(future_impl.dart:190:18)
at handleWhenCompleteCallback (future_impl.dart:736:38)
at _Future._propagateToListeners (future_impl.dart:792:11)
at [_completeWithValue] (future_impl.dart:566:5)
at future.dart:527:22
at _RootZone.runUnary (zone.dart:1685:54)
at _FutureListener.then.handleValue (future_impl.dart:147:18)
at handleValueCallback (future_impl.dart:766:44)
at _Future._propagateToListeners (future_impl.dart:795:13)
at async._AsyncCallbackEntry.new.callback (future_impl.dart:430:9)
at Object._microtaskLoop (schedule_microtask.dart:40:11)
at _startMicrotaskLoop (schedule_microtask.dart:49:5)
at async_patch.dart:166:15
```
This also happens in production, but exceptions are not visible there. User
experience is not impacted.
Cause:
At this line:
https://github.com/apache/beam/blob/0ef5d3a185c1420da118208353ceb0b40b3a27c9/playground/frontend/lib/pages/playground/components/playground_page_providers.dart#L62
`PlaygroundState` object is re-created when anything in `ExampleState` (the
snippet directory) changes. I have seen it happening 6 times at startup. This
leads to the old `PlaygroundState`s being disposed in a split second after
being created.
However, a few lines after that we call `setExample` on them after the
asynchronous gap. So this call often happens when the object is already
disposed.
### Issue Priority
Priority: 3
### Issue Component
Component: beam-playground
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