Javascript source map errors when debugging

Describe the bug
Browsers report Javascript source map errors because of missing or inaccessible map files or original source file is actually missing when specified.

To Reproduce
Steps to reproduce the behavior:

1. Start OpenRefine
2. In Firefox browser, use it's console tool.
3. See error -->

Current Results

Source map error: Error: request failed with status 404
Resource URL: http://127.0.0.1:3333/index-bundle.js
Source Map URL: jquery.contextMenu.min.js.map

Expected behavior
No Source map errors with 404 code (referencing OpenRefine provided JS files).
Source maps should either be provided and accessible through Jetty or the sourceMappingURL comment should be removed from JS files when not provided.
Such as :
//# sourceMappingURL=jquery.contextMenu.min.js.map
//# sourceMappingURL=underscore-min.map

Screenshots

Desktop:

• OS: Windows 10
• Browser Version: Firefox, Edge
• JRE or JDK Version: JRE 1.8.0_181

OpenRefine :

• Version OpenRefine 3.5 Beta

Additional context
https://developer.mozilla.org/en-US/docs/Tools/Debugger/Source_map_errors
https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Use_a_source_map

Start by locating the generated index-bundle.js and the jquery.contextMenu.min.js and underscore-min.js assets referenced by the report. Check how their source maps are exposed through Jetty or whether the sourceMappingURL comments point to missing files; done means Firefox and Edge no longer report 404 source-map errors for OpenRefine JavaScript files.