fix: fix a popState race-condition (#12925)

When loading a non-SSR page and pressing the browser's back button while the page is still loading, it's possible for `current.url` to still be the default value of `null`, which then causes the `popState` function to throw. We therefore guard against that. Fixes #12924

Author: aloisklink

.changeset/angry-months-speak.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: fix race-condition when not using SSR when pressing back before initial load

packages/kit/src/runtime/client/client.js

@@ -2330,7 +2330,7 @@ function _start_router() {
 const state = event.state[STATES_KEY] ?? {};
 const url = new URL(event.state[PAGE_URL_KEY] ?? location.href);
 const navigation_index = event.state[NAVIGATION_INDEX];
-const is_hash_change = strip_hash(location) === strip_hash(current.url);
+const is_hash_change = current.url ? strip_hash(location) === strip_hash(current.url) : false;
 const shallow =
 navigation_index === current_navigation_index && (has_navigated || is_hash_change);