how to solve service_worker_en.js:3 Uncaught (in promise) TypeError: Failed to execute 'fetch' on 'WorkerGlobalScope'?

My Chrome console is giving this error for my website

service_worker_en.js:3 Uncaught (in promise) TypeError: Failed to execute 'fetch' on 'WorkerGlobalScope': Cannot construct a Request with a Request object that has already been used.

I have read this article on S.O but still can not figure out what the problem really is with my code. Here is my service worker code

self.addEventListener("install", function(event) {
    event.waitUntil(preLoad());
});
var preLoad = function() {
    console.log("Installing web app");
    return caches.open("offline").then(function(cache) {
        console.log("caching index and important routes");
        return cache.addAll(["/", "/devotional-language-en-tab-9"]);
    });
};
self.addEventListener("fetch", function(event) {
    event.respondWith(checkResponse(event.request).catch(function() {
        return returnFromCache(event.request);
    }));
    event.waitUntil(addToCache(event.request));
});
var checkResponse = function(request) {
    return new Promise(function(fulfill, reject) {
        fetch(request).then(function(response) {
            if (response.status !== 404) {
                fulfill(response);
            } else {
                reject();
            }
        }, reject);
    });
};
var addToCache = function(request) {
    return caches.open("offline").then(function(cache) {
        return fetch(request).then(function(response) {
            console.log(response.url + " was cached");
            return cache.put(request, response);
        });
    });
};
var returnFromCache = function(request) {
    return caches.open("offline").then(function(cache) {
        return cache.match(request).then(function(matching) {
            if (!matching || matching.status == 404) {
                return cache.match("offline_en.html");
            } else {
                return matching;
            }
        });
    });
};

According to Chrome the error is mainly on this two lines :

how to solve service_worker_en.js:3 Uncaught (in promise) TypeError: ?