Set an empty tocken, not null

src/App.js

@@ -71,7 +71,7 @@ function renewToken(oldToken, setToken) {
     })
     .catch((error) => {
       if (error instanceof ResponseError && error.response.status === 401) {
-        setToken(null);
+        setToken("");
       } else {
         console.log("Error renewing token: " + error.message);
       }