DevTools failed to load SourceMap: Could not load content for http://localhost/4-es2015.js.map: Connection error: net::ERR_CONNECTION_REFUSED code example

Example: DevTools failed to load SourceMap: Could not load content for

Open Developer Tools, 
go to "Settings" for Developer Tools, 
then uncheck Enable JavaScript Sourcemaps under the "Sources" settings.
Restart Browser if needed