[javascript] DevTools failed to load SourceMap: Could not load content for chrome-extension

For me, the problem was caused not by the app in development itself but by the Chrome extension: React Developer Tool. I solved partially that by right-clicking the extension icon in the toolbar, clicking "manage extension" (I'm freely translating menu text here since my browser language is in Brazilian Portuguese), then enabling "Allow access to files URLs." But this measure fixed just some of the alerts.

I found issues in the react repo that suggests the cause is a bug in their extension and is planned to be corrected soon - see issues 20091 and 20075.

You can confirm is extension-related by accessing your app in an anonymous tab without any extension enabled.