error - add list items to the static class variable C#

As already mentioned in my first reply that variable needs to be initialized in memory to access its methods.Thats the reason of throwing 'Object reference not set to an instance of an object'. As it is prefix by static keyword, you can access classname.variablename directly. :)

This happens just because you had not initialozed your static variables using new keyword and without being initialized you are adding value to it