added empty return

Signed-off-by: Andre Wanlin <67169551+awanlin@users.noreply.github.com>

.changeset/fluffy-pens-prove.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Updated `frameHandler` to return `undefined` when using the redirect flow instead of returning `postMessageReponse` which was causing errors

plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts

@@ -187,6 +187,7 @@ export class OAuthAdapter implements AuthProviderRouteHandlers {
           );
         }
         res.redirect(state.redirectUrl);
+        return undefined;
       }
       // post message back to popup if successful
       return postMessageResponse(res, appOrigin, responseObj);