WCF Error as object reference not set to an instance of an object

That is because either of your parameters are null. For me, the second parameter has a more chance of being null; because usually you've already filled the ConnectionString property, or either of it, the connection string is not available.

So, if you're using Visual Studio, debug the code having a breakpoint set at this position. Then once your code reaches this location, you will see which of the cases is true; which variable is null in your code. Then you can (above that particular line of invoking the base-class's constructor) set the value to a non-null value to minimize this problem.

Remember, WCF also runs over .NET framework, so an error that it would throw would be easily handled as all other .NET exceptions. NullReferenceException[^] is an exception of .NET framework, and beginners always fall for it as it is a bit tricky to find which object threw this, and why it was thrown. Good luck.