inTheHand Blutooth_Client.DiscoverDevices() reports "System.NullReferenceException: 'Object reference not set to an instance of an object.'"

I attempted to write the Server code and Client code using inTheHand.Net.Bluetooth to promote communication between android watch and Windows. However, the line "client.DiscoverDevices();" keeps recording the following error. Is it possible for anybody to have a look and give some help? Really appreciate it if anyone can help by any amount.

The error message is:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Occured at the line:

Bluetooth client = new BluetoothClient();
for each (var item in client.DiscoverDevices()) {
    ......
}

Such a problem will not occur when running PC-PC communication program using 32Feet.net package.