fix(clerk-js): Use default value for signIn.userData

.changeset/gorgeous-pans-pump.md

@@ -0,0 +1,5 @@
+---
+'@clerk/clerk-js': patch
+---
+
+Use default value for `signIn.userData`. This will prevent the SignIn modal from breaking when an appearance prop is used in Next.js App Router.

packages/clerk-js/src/core/resources/SignIn.ts

@@ -65,7 +65,7 @@ export class SignIn extends BaseResource implements SignInResource {
   secondFactorVerification: VerificationResource = new Verification(null);
   identifier: string | null = null;
   createdSessionId: string | null = null;
-  userData!: UserData;
+  userData: UserData = new UserData(null);
 
   constructor(data: SignInJSON | null = null) {
     super();

packages/clerk-js/src/core/resources/UserData.ts

@@ -6,7 +6,7 @@ export class UserData implements IUserData {
   imageUrl?: string;
   hasImage?: boolean;
 
-  constructor(data: UserDataJSON) {
+  constructor(data: UserDataJSON | null) {
     this.fromJSON(data);
   }