Fixed empty object getting called

ref #99

src/models/actions/create/CreateUser.ts

@@ -95,7 +95,7 @@ export class CreateUser {
         if (!this.email) {
             throw new UserEmailNeededError();
         }
-        if (this.username.includes("@")) { throw new UsernameContainsIllegalCharacterError(); }
+        if (this.username?.includes("@")) { throw new UsernameContainsIllegalCharacterError(); }
 
         let password_strength = passwordStrength(this.password);
         if (!password_strength.contains.includes("uppercase")) { throw new PasswordMustContainUppercaseLetterError(); }