Hi Ellen,
I also got the issue while building the widget, I found a workaround for resolving the error.
Open the file which got error. In your case it is in "node_modules/react-leaflet-custom-control/lib/Control.js (1:15)"
Scroll to the bottom of the file where you will find this source map line "# sourceMappingURL".
Remove that line, Save the file and build again.
I hope the above workaround helps.