Fix KeeWebHTTP plugion Firefox compatibility

docs/plugins/keewebhttp/manifest.json

@@ -1,5 +1,5 @@
 {
-  "version": "1.0.1",
+  "version": "1.0.2",
   "manifestVersion": "0.1.0",
   "versionMin": "1.11.0",
   "name": "keewebhttp",

docs/plugins/keewebhttp/plugin.js

@@ -66,7 +66,7 @@ function run() {
         server = http.createServer((req, res) => {
             const origin = req.headers.origin;
             const referer = req.headers.referrer || req.headers.referer;
-            if (req.method !== 'POST' || referer || origin &&
+            if (req.method !== 'POST' || referer || origin === null || origin &&
                 !origin.startsWith('chrome-extension://') && !origin.startsWith('safari-extension://')
             ) {
                 if (DebugMode) {