PHP variable returning null. (Laravel)

Am calling function generateUniqueTinyCode() from another function create(). it returns a generated code. After execution sometimes it returns as null (in some cases) even though the variable has value. Can anyone help me?

The var_dump($this->randomCode) just before returning function has value. but after that dd($Code) is shown as null.

Note : This is a Facade ( MyTiny ).So i can call MyTiny::create('https://laravel.com/docs/5.7);

public function create( $url = null ){
    if(self::validateURL($url)){    
        $tinyCode = $this->generateUniqueTinyCode($this->getMinLength());
        dd($tinyCode);
        exit;

        DB::table(self::getTableName())->insert([
            'short_code' => $tinyCode,
            'short_url'  => url($this->url_prefix.'/'.$tinyCode), 
            'long_url'   => $url,
            'created_at' => date("Y-m-d H:i:s"),
            'updated_at' => date("Y-m-d H:i:s")
        ]);
        return url($this->url_prefix.'/'.$tinyCode);
    } else {

        return "Please input or specify a valid URL!";
    }

}

private function generateUniqueTinyCode($length = 2) {
    if(config('tinyurls.USE_NUMBERS')){
        $characters = '21';
    } else {
        $characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    }

    $charactersLength = strlen($characters);
    $this->randomCode = '';
    for ($i = 0; $i < $length; $i++) {
        $this->randomCode .= $characters[rand(0, $charactersLength - 1)];
    }


     // Checks code already generated
    $codeAlreadyExist = DB::table(self::getTableName())->where('short_code', '=', $this->randomCode)->exists();
    if($codeAlreadyExist){
         $this->generateUniqueTinyCode($this->getMinLength());
    } else {

      var_dump($this->randomCode);
       return $this->randomCode;
    }   

}

OUTPUT

string(2) "21"
null

via Chebli Mohamed