In the WSN Configuration.xml ICON PATH field is missing in the Device Properties.
To get the Configuration file to open you will have to add it manually in the configuration file.
Depending on your NetSim Installation path which is usually like "C:\Program Files (x86)\NetSim Standard\" you will have to add a field in each of the device in the Configuration.xml.
Open your sample configuration file and add the following in the respective device properties just before the Device Id

For Sensors:
DEVICE_ICON_PATH="C:\Program Files (x86)\NetSim Standard\Docs\Icons\SensorDrop.png"

For Sink Node:
DEVICE_ICON_PATH="C:\Program Files (x86)\NetSim Standard\Docs\Icons\SinkNodeDrop.png"

For Agents:
ICON_PATH="C:\Program Files (x86)\NetSim Standard\Docs\Icons\AgentDrop.png"

You will not face this problem in the scenarios that you create and save in WSN, This issue exists only in the Sample Configuration file.