add missing verifier check

Co-authored-by: Copilot <[email protected]>

Author: patrislav

packages/wallet/wdk/src/sequence/handlers/authcode-pkce.ts

@@ -60,8 +60,10 @@ export class AuthCodePkceHandler extends AuthCodeHandler implements Handler {
     code: string,
   ): Promise<[IdentitySigner, { [key: string]: string }]> {
     const challenge = new Identity.AuthCodePkceChallenge('', '', '')
-    const signer = await this.nitroCompleteAuth(challenge.withAnswer(commitment.verifier ?? '', code))
-
+    if (!commitment.verifier) {
+      throw new Error('Missing verifier in commitment')
+    }
+    const signer = await this.nitroCompleteAuth(challenge.withAnswer(commitment.verifier, code))
     await this.commitments.del(commitment.id)
 
     return [signer, commitment.metadata]