NullReferenceException when adding String to list

Hey guys! I have a List of strings which i call “data”, and i have a string that i call “hint”. I am trying to verify if data DOESN’T contain hint so i can add it to the list, but i am getting that boring NullReferenceException on this part of the code:

                if (!data.Contains(hint)) // <- THIS PART (LINE 27)

What may be wrong? If i had any hint (good old irony) of what was happening i would fix it, but instead i just get this error…

Well data is null. Wherever you declared it, you didn’t assign it a new List