Error with URL in NextJS and I can't find the problem - reactjs

Question

I have an error that appear in both development and production mode, however, this error only crash the website in production.
I have this error returned:
unhandledRejection: TypeError: Only absolute URLs are supported
at getNodeRequestOptions (D:\FIV\nextjs-fiv-4\node_modules\node-fetch\lib\index.js:1305:9)
at D:\FIV\nextjs-fiv-4\node_modules\node-fetch\lib\index.js:1410:19
at new Promise (<anonymous>)
at Function.fetch [as default] (D:\FIV\nextjs-fiv-4\node_modules\node-fetch\lib\index.js:1407:9)
at fetchWithAgent (D:\FIV\nextjs-fiv-4\node_modules\next\dist\server\node-polyfill-fetch.js:38:39)
at _callee$ (D:\FIV\nextjs-fiv-4\node_modules\swell-js\dist\api.js:191:20)
at tryCatch (D:\FIV\nextjs-fiv-4\node_modules\regenerator-runtime\runtime.js:63:40)
at Generator.invoke [as _invoke] (D:\FIV\nextjs-fiv-4\node_modules\regenerator-runtime\runtime.js:294:22)
at Generator.next (D:\FIV\nextjs-fiv-4\node_modules\regenerator-runtime\runtime.js:119:21)
at asyncGeneratorStep (D:\FIV\nextjs-fiv-4\node_modules\swell-js\node_modules\#babel\runtime\helpers\asyncToGenerator.js:3:24)
And I know that this is the element below causing this error, when I remove it, the error disappears. But I do not understand how to correct it, or what's wrong when checking the network tab. In the network tab I see that this is calling http://localhost:3000/null/api/cart instead of https://my-store.swell.store/api/cart which is weird because on other page this URL is called correctly and I use an .env file to set my environment variables.
//Render Stripe Card Element
var customCardElement = swell.payment.createElements({
card: {
elementId: '#card-element', // default: #card-element
options: {
style: {
base: {
fontWeight: 500,
fontSize: "16px"
},
},
},
onChange: event => {
//setDisabled(event.empty);
if(event.error){
//setError(event.error.message);
}
}
}
});

Answer 1

Client side NextJS variables are only available if prefixed with NEXT_PUBLIC_ because well... they're public