state.js: do not call an undefined function

When changing page in a stateless app, there is no state to set `is_hydrated: False`

reflex/.templates/web/utils/state.js

@@ -646,7 +646,12 @@ export const useEventLoop = (
 
   // Route after the initial page hydration.
   useEffect(() => {
-    const change_start = () => dispatch["state"]({is_hydrated: false})
+    const change_start = () => {
+      const main_state_dispatch = dispatch["state"]
+      if (main_state_dispatch !== undefined) {
+        main_state_dispatch({is_hydrated: false})
+      }
+    }
     const change_complete = () => addEvents(onLoadInternalEvent());
     router.events.on("routeChangeStart", change_start);
     router.events.on("routeChangeComplete", change_complete);