Getting warning "The source expression is always of pattern's type, matches on all non-null values"

You're right!

ReSharper (not Visual Studio) is factually right, although I don't know why that would be a warning.

Although Customers is a collection of Customer, the use of SingleOrDefault hints that the value might be null which is not a Customer.

And nothing says that all values coming out of Customers are non-null.