bug: fix ReferenceError

[TomHodson]

As described in issue #827 running

npm install
npm run build

And then serving the resulting dist folder, results in a page that displays a never ending loading spinner and the console displays:

  Uncaught ReferenceError: L is not defined                                                                                                                                                                                                                                                                       
      <anonymous> GeoJsonNoVanish.ts:1                                                                                                                                                                                                                                                                            
  GeoJsonNoVanish.ts:1:1   

I believe the fix is likely something along the lines of this PR. I have tested this change locally.

Oddly it all seems to work fine with npm run dev, it's on the built dist which is shows the problem.