Well, I cannot see your html so this is just speculation on my part...
Do you have 11 TextBox controls named txt0, txt1, txt2, txt3, txt4, txt5, txt6, txt7, txt8, txt9, and txt10?
I notice you are starting you i at 0 not 1 so you must have a txt0.
And they must be named exactly "txt" + all options of i or you will get a null Object error like you have gotten.
Another note in the code, maybe you have stripped something out but:
StringCollection sc = new StringCollection();
This is declared inside of the for block and will not be available for use after it closes. I do not see you doing anything with it in the code given so I must assume you intend to do something with it after the for loop, so you need to declare the sc variable before you open the for loop.
This is variable "scope" and right now the scope is only the for loop itself.