Merge pull request #17015 from backstage/auth/fix-guest-custom

Fix guest/custom auth

.changeset/all-my-auth-wanna-ride-today.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-components': patch
+---
+
+Fixed a bug that could prevent auth from working when using the `guest` or `custom` auth providers.

packages/core-components/src/layout/SignInPage/providers.tsx

@@ -177,7 +177,10 @@ export const useSignInProviders = (
         };
 
         const handleSignInStarted = () => {
-          localStorage.setItem(PROVIDER_STORAGE_KEY, provider.config!.id);
+          localStorage.setItem(
+            PROVIDER_STORAGE_KEY,
+            provider?.config?.id || provider.id,
+          );
         };
 
         const handleSignInFailure = () => {