PHP Notice: Undefined index

Hi Gavick,
This is my first time using this forum. Hope my english is enough for you to understand, even I know it's poor
I've used yourshop for several weeks on my client's site (hoatay.net). When i log in to File Manager on the hosting, I found the error log file is created and the file size is getting bigger and bigger.
I download it and found there is only one error recorded. Here it is:

[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/gk.template.helper.php on line 368
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/mobile_device_detect.php on line 4
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/gk.template.helper.php on line 368
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/mobile_device_detect.php on line 4
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/gk.template.helper.php on line 266
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/gk.template.helper.php on line 368
[06-Apr-2012 21:25:12] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/hoatayne/public_html/templates/hoatayhandmade/libs/mobile_device_detect.php on line 4
[06-Apr-2012 14:25:41] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 14:25:41] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 14:26:27] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 14:26:27] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 14:27:04] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73
[06-Apr-2012 14:27:04] PHP Notice: Undefined index: option in /home/hoatayne/public_html/templates/hoatayhandmade/layouts/blocks/main.php on line 73

You can view the detail of the error file on the attached one.
I haven't touched any code of the template yet, just only increase the template width.
How to fix this? Will it cause any harm to the site?
Thanks!

Any browser or user-agent can choose not to send the User-Agent header. If they do not send that header, $_SERVER['HTTP_USER_AGENT'] won't be set and that might be the cause of this error. I have checked your website and it is version v1.03 joomla 1.5 which files are not available any more. If you can attach following files in a zip file so we can check please.
/public_html/templates/hoatayhandmade/libs/gk.template.helper.php
/public_html/templates/hoatayhandmade/libs/mobile_device_detect.php
/public_html/templates/hoatayhandmade/layouts/blocks/main.php

See you around...

Thank you for your replying Norman.
There is another folder contains the latest version, but it cause some problem display product so I go back to 1.03. You can find it here: templates/gk_yourshop.
Please check the zip file to see the 3 files you request:
/public_html/templates/hoatayhandmade/libs/gk.template.helper.php
/public_html/templates/hoatayhandmade/libs/mobile_device_detect.php
/public_html/templates/hoatayhandmade/layouts/blocks/main.php

I notice that the error still there even when i replaced these 3 files with the ones from the latest version.