NullPiotrException
**Description** When changing username, if we use uppercase characters, we get an error `Nazwa użytkownika zawiera niedozwolone znaki specjalne.` (ang.: `Username contains forbidden special characters.`), even though only letters were used, which are not special characters. **Reproduction steps** 1. Log in to inFakt. 2. Click gear icon in the upper right corner. 3. Click `Konto inFakt`. 4. In the `Dane dostępowe` section click `Edytuj`. 5. In the `Nazwa użytkownika` field enter at least 1 uppercase characters and at least 4 lowercase characters. 6. In the `Hasło` field enter your password. 7. Click `Zapisz` button. **Expected behavior** Error message mentioning that uppercase characters are not allowed. Additionally, it would be a good idea UX-wise to display the requirements (5-250 lowercase characters) before sending the form. **Screenshots / videos**  **Environment:** - Firefox 140.0b9 - Chrome 138.0.7204.35