STR: - open https://treeherder.mozilla.org/#/jobs?repo=try&revision=9e27545a9fee79ef2be628721170289ac01f00b3&selectedJob=203976072 - select node `#keyboard-shortcuts` in the inspector - in the layout sidebar, click on the toggle to activate the flexbox highlighter (not sure this is actually needed to repro the crash) - make sure you have the flex item accordion expanded too - now in the inspector, select the parent element `.d-flex.flex-column.h-100` ==> the layout tab crashes. The following errors are displayed in the browser console: TypeError: "flexItem is undefined; can't access its "nodeFront" property" render resource://devtools/client/inspector/flexbox/components/FlexItemSelector.js:73:11 finishClassComponent resource://devtools/client/shared/vendor/react-dom.js:10332:22 updateClassComponent resource://devtools/client/shared/vendor/react-dom.js:10295:10 beginWork resource://devtools/client/shared/vendor/react-dom.js:10894:14 performUnitOfWork resource://devtools/client/shared/vendor/react-dom.js:12713:12 workLoop resource://devtools/client/shared/vendor/react-dom.js:12730:24 renderRoot resource://devtools/client/shared/vendor/react-dom.js:12770:7 performWorkOnRoot resource://devtools/client/shared/vendor/react-dom.js:13359:22 performWork resource://devtools/client/shared/vendor/react-dom.js:13281:7 performSyncWork resource://devtools/client/shared/vendor/react-dom.js:13253:3 requestWork resource://devtools/client/shared/vendor/react-dom.js:13153:5 1 resource://devtools/client/shared/vendor/react-dom.js:13022:11 enqueueSetState resource://devtools/client/shared/vendor/react-dom.js:8845:5 setState resource://devtools/client/shared/vendor/react.js:406:3 onStateChange resource://devtools/client/shared/vendor/react-redux.js:1412:11 dispatch resource://devtools/client/shared/vendor/redux.js:416:7 waitUntilService resource://devtools/client/shared/redux/middleware/wait-service.js:59:20 promiseMiddleware resource://devtools/client/shared/redux/middleware/promise.js:16:14 thunk resource://devtools/client/shared/redux/middleware/thunk.js:16:9 task resource://devtools/client/shared/redux/middleware/task.js:39:12 update resource://devtools/client/inspector/flexbox/flexbox.js:451:7 react-dom.js:11273:5 ​

Pushed by gabriel.luong@gmail.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/e68617c25cbe Ensure a valid flex item prop is provided to the FlexItemSelector. r=pbro