when updating the inspector to use getFront, browser_ext_devtools_panels_elements_sidebar.js fails. We are sending events to the toolbox after the toolbox is closed, and everything is asynchronous. In this case specifically, the "removeTab" action in ToolBar --> https://searchfox.org/mozilla-central/source/devtools/client/shared/components/tabs/TabBar.js#205-227 which sends three "select" functions just as the toolbox is closing. Since destroy functions are asynchronous, halfway in the "update" that is triggered by this select, we lose the reference to the inspector or the store. The issue occures in both the grid and the flex box. we have a try/catch block that is async: https://searchfox.org/mozilla-central/rev/05d91d3e02a0780f44599371005591d7988e2809/devtools/client/inspector/grids/grid-inspector.js#242-248 so this then fails: https://searchfox.org/mozilla-central/rev/05d91d3e02a0780f44599371005591d7988e2809/devtools/client/inspector/grids/grid-inspector.js#251-253 and by the time this resolves the toolbox is already destroyed. I have introduced a try/catch block for the lines 251-253, but we should find a better way to handle destruction.