Update gm_api.ts

Author: cyfung1031

src/app/service/content/gm_api/gm_api.ts

@@ -1409,6 +1409,7 @@ export default class GMApi extends GM_Base {
         error = null; // GC
       };
       const onStart = async (con: MessageConnect) => {
+        if (killConn === null || done) return;
         try {
           result = await cb();
         } catch (e) {