Right: it has nothing to do with your code. I've found two valid solutions to this warning (not just disabling it). To better understand what a SourceMap is, I suggest you check out this answer, where it explains how it's something that helps you debug:
The .map files are for js and css (and now ts too) files that have been minified. They are called SourceMaps. When you minify a file, like the angular.js file, it takes thousands of lines of pretty code and turns it into only a few lines of ugly code. Hopefully, when you are shipping your code to production, you are using the minified code instead of the full, unminified version. When your app is in production, and has an error, the sourcemap will help take your ugly file, and will allow you to see the original version of the code. If you didn't have the sourcemap, then any error would seem cryptic at best.
First solution: apparently, Mr Heelis was the closest one: you should add the .map file and there are some tools that help you with this problem (Grunt, Gulp and Google closure for example, quoting the answer). Otherwise you can download the .map file from official sites like Bootstrap, jquery, font-awesome, preload and so on.. (maybe installing things like popper or swiper by the npm command in a random folder and copying just the .map file in your js/css destination folder)
Second solution (the one I used): add the source files using a CDN (here all the advantages of using a CDN). Using the Content delivery network (CDN) you can simply add the cdn link, instead of the path to your folder. You can find cdn on official websites (Bootstrap, jquery, popper, etc..) or you can easily search on some websites like cloudflare, cdnjs, etc..