getUserName(): Return value must be of type string, null returned

User Model

    public function getFilamentName(): string
    {
        return $this->username ?: 'Unknown User';
    }

    public function getFilamentName(): string
    {
        return $this->username ?: 'Unknown User';
    }

    public function getFilamentName(): string
    {
        return $this->username ?: 'Unknown User';
    }

    public function getFilamentName(): string
    {
        return $this->username ?: 'Unknown User';
    }

ERROR

Filament\FilamentManager::getUserName(): Return value must be of type string, null returned

Filament\FilamentManager::getUserName(): Return value must be of type string, null returned

Filament\FilamentManager::getUserName(): Return value must be of type string, null returned

Filament\FilamentManager::getUserName(): Return value must be of type string, null returned