how do I find out where this warning come from and mitigate the reason?
2024-11-05 19:44:23.628 [WARN ] [e.internal.SseItemStatesEventBuilder] - Attempting to send a state update of an item which doesn’t exist: undefined
That warning is from the UI and in nearly all instances is coming from a custom widget. Somewhere you have an expression in a field for one of your widgets and that expression is evaluating to undefined instead of a valid item name. It is just a warning and is not impacting the performance of your system (other than the fact that you have a widget that might not work properly).
There is no direct way to trace that warning back to the widget that is causing it. You can narrow it down by noting which page load causes the message to appear and then check the configurations and code of the widgets on that page to see which ones have an expression defining a property that should be an item name.
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Knowing that is coming from a widget helped a lot 