Dear Friend,
An object reference not set to an instance of an object comes whenever you are using or assigning a variable to something which is not yet initialized and what i get from your line of code error is that your connection string
ConfigurationManager.AppSettings["ConnectionString"].ToString();
is blank as you may not have initialized it in the webconfig file.
So try to check there you may get the solution there.
If it helps you out then mark this as your answer and i am open to queries.
Thanks