# \_args.Sender error

**URL:** <https://forums.fusetools.com/t/args-sender-error/4482>\
**Category:** General\
**Created:** [January 20, 2016, 11:28am UTC](https://forums.fusetools.com/t/args-sender-error/4482 "2016-01-20T11:28:28Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Fahad](https://avatars.discourse-cdn.com/v4/letter/f/ecccb3/32.png) [@Fahad](https://forums.fusetools.com/u/Fahad)\
**Post date:** [January 20, 2016, 11:28am UTC](https://forums.fusetools.com/t/args-sender-error/4482/1 "2016-01-20T11:28:28Z")

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I’m getting this error:

```auto
ERROR: _args.Sender was null
    System.Exception occured.
       at Fuse.Reactive.CrossThreadExceptionHandler.CheckAndThrow() in C:\ProgramData\Uno\Packages\Fuse.Reactive\0.18.8\$.uno:line 1540
       at Fuse.Reactive.JavaScript.HandleWorkerException() in C:\ProgramData\Uno\Packages\Fuse.Reactive\0.18.8\$.uno:line 1580
       at Fuse.UpdateListener.Invoke() in C:\ProgramData\Uno\Packages\FuseCore\0.18.8\$.uno:line 6596
       at Fuse.UpdateManager.Update(Stage stage) in C:\ProgramData\Uno\Packages\FuseCore\0.18.8\$.uno:line 6817
LOG: One or more errors occurred.

```

It happens generally when I’m clearing an observable list and replacing it’s contents, but, sometimes this happens literally after a few seconds when I don’t do anything. I think its specific to my project, but I can just press Restart and then try again and it’ll work but it can take a few restarts.

This also never happens when I build to device (not preview).

Cheers

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<div class="post-metadata">

**Author:** ![jveres](https://yyz1.discourse-cdn.com/flex035/user_avatar/forums.fusetools.com/jveres/32/545_2.png) [@jveres](https://forums.fusetools.com/u/jveres)\
**Post date:** [January 20, 2016, 11:51am UTC](https://forums.fusetools.com/t/args-sender-error/4482/2 "2016-01-20T11:51:04Z")

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Without knowing the JS source for me it seems like a possible async workers issue ex. clearing an observable in a timer while still processing it in another async loop. The way I’m using observable list is to put a flag ahead of processing and unflag when done (like a semaphore). I haven’t seen that exception yet though. Hope it helps.

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**Author:** ![Anders\_Bondehagen](https://yyz1.discourse-cdn.com/flex035/user_avatar/forums.fusetools.com/anders_bondehagen/32/98_2.png) [@Anders\_Bondehagen](https://forums.fusetools.com/u/Anders_Bondehagen)\
**Post date:** [January 20, 2016, 12:52pm UTC](https://forums.fusetools.com/t/args-sender-error/4482/3 "2016-01-20T12:52:12Z")

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Do you have a project that we can use to reproduce this? Are you on Mac or Windows?

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<div class="post-metadata">

**Author:** ![Fahad](https://avatars.discourse-cdn.com/v4/letter/f/ecccb3/32.png) [@Fahad](https://forums.fusetools.com/u/Fahad)\
**Post date:** [January 20, 2016, 1:09pm UTC](https://forums.fusetools.com/t/args-sender-error/4482/4 "2016-01-20T13:09:38Z")

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I can’t isolate it. There’s a lot of observables being manipulated at the same time, and there is a timer running, but it isn’t/shouldn’t be touching any observable list.

I don’t want to release the source publicly.

I’m on Windows.

UPDATE

When I check the observable list just before the app crashes, every single subscriber is null, I’m not sure what that means or how I can prevent it, but I think this is where it’s going wrong

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<div class="post-metadata">

**Author:** ![Anders\_Bondehagen](https://yyz1.discourse-cdn.com/flex035/user_avatar/forums.fusetools.com/anders_bondehagen/32/98_2.png) [@Anders\_Bondehagen](https://forums.fusetools.com/u/Anders_Bondehagen)\
**Post date:** [January 20, 2016, 4:54pm UTC](https://forums.fusetools.com/t/args-sender-error/4482/5 "2016-01-20T16:54:54Z")

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This happened when the observable values was replaced using clear and then add, instead of using replaceAll. So the nodes was unrooted and the event could not execute. I have added a better errormessage to handle this case differently in the future.