fetch blob instead of window.url to download file

src/views/feature_type/Feature_type_detail.vue

@@ -340,15 +340,15 @@ export default {
 
     exportFeatures() {
       const url = `${this.$store.state.configuration.VUE_APP_DJANGO_API_BASE}projects/${this.$route.params.slug}/feature-type/${this.$route.params.feature_type_slug}/export/`;
-      featureAPI
-        .getFeaturesBlob(url)
-        .then((blob) => {
+      featureAPI.getFeaturesBlob(url).then((blob) => {
+        if (blob) {
           const link = document.createElement("a");
           link.href = URL.createObjectURL(blob);
           link.download = `${this.project.title}-${this.structure.title}.json`;
           link.click();
           URL.revokeObjectURL(link.href);
-        })
+        }
+      });
     },
   },