The issue is noticed in the browser when doing development.
We normally use Chrome for development (Version 83.0.4103.97 (Official Build) (64-bit), this morning I tried Firefox and the issue does not occur in Firefox.
I happen when you want to edit a template or adaptive form or site, just anything once you go into edit mode it will appear. Sometimes 1 error, sometimes 3, occasionally the error appears 5 times at one go.
granite.js:259 [Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/. structure.html:1 [DOM] Found 2 elements with non-unique id #editor-styleselector-form: (More info: https://goo.gl/9p2vKq)
6core.js:647 Handler of component is invalid -> TypeError: Cannot read property 'flow' of undefined at eval (eval at ns.util.sanitizeCQHandler (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:272:94), <anonymous>:1:14) at Object.ns.util.sanitizeCQHandler (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:272:94) at String.<anonymous> (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:322:337) at Function.each (https://localhost:4502/etc.clientlibs/clientlibs/granite/jquery.js:377:19) at constructor._loadConfig (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:321:277) at constructor.updateConfig (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:323:64) at Object.<anonymous> (https://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/core.js:922:131) at fire (https://localhost:4502/etc.clientlibs/clientlibs/granite/jquery.js:3233:31) at Object.fireWith [as resolveWith] (https://localhost:4502/etc.clientlibs/clientlibs/granite/jquery.js:3363:7) at done (https://localhost:4502/etc.clientlibs/clientlibs/granite/jquery.js:9841:14) contexthub:442 Uncaught TypeError: Cannot read property 'externalize' of undefined at contexthub:442 at contexthub:446 structure.html:94 Uncaught TypeError: window.injectContextHubUI is not a function at structure.html:94 at structure.html:127 6