Fix http getter return format.

src/app/_helpers/http-getter.ts

@@ -6,7 +6,7 @@ HttpGetter.prototype.get = (filename) =>
   new Promise((resolve, reject) => {
     fetch(filename).then((response) => {
       if (response.ok) {
-        resolve(response.json());
+        resolve(response.text());
       } else {
         reject(rejectBody(response));
       }