I'm also confused because that ref gets nulled out [here](https://searchfox.org/mozilla-central/rev/6444ed17e9f5e0d4e7dabc13c930d46b529fca15/browser/components/customizableui/PanelMultiView.jsm#453) which is called [here](https://searchfox.org/mozilla-central/rev/6444ed17e9f5e0d4e7dabc13c930d46b529fca15/browser/components/customizableui/PanelMultiView.jsm#362) when the window closes. Does that not happen?
Bug 1816693 Comment 5 Edit History
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I'm also confused because that ref (ie to `_openPopupCancelCallback`) gets nulled out [here](https://searchfox.org/mozilla-central/rev/6444ed17e9f5e0d4e7dabc13c930d46b529fca15/browser/components/customizableui/PanelMultiView.jsm#453) which is called [here](https://searchfox.org/mozilla-central/rev/6444ed17e9f5e0d4e7dabc13c930d46b529fca15/browser/components/customizableui/PanelMultiView.jsm#362) when the window closes. Does that not happen?