double is a value type. value types dont really have null values. but you can have nullable double by using this syntax.
Nullable<Double> d = null;
this will work same as a double. try using it. Just remember nullable types are immutable. but since you are using double it wont be a big problem for you.
P.S. the shorthand of what i just told you is like
Double? d = null;
it means the same. just a syntactic sugar for the first one.