Check if single() LINQ return NULL

Use SingleOrDefault instead.

Single throws an exception when the enumeration does not contain exactly one element, SingleOrDefault<T> returns default(T) (which is null for reference types) when called on empty enumerations instead. Note that both will throw if there is more than one element in the enumeration.

By design, Single will throw an exception when the sequence is empty. Use SingleOrDefault to return null when your sequence is empty.