Opcua server never exits

sample-server.js

/*global require,setInterval,console */
const opcua = require("node-opcua");

// Let's create an instance of OPCUAServer
const server = new opcua.OPCUAServer({
  port: 4334, // the port of the listening socket of the server
  resourcePath: "/UA/MyLittleServer", // this path will be added to the endpoint resource name
  buildInfo: {
    productName: "MySampleServer1",
    buildNumber: "7658",
    buildDate: new Date(2014, 5, 2)
  }
});

async function post_initialize() {
  return new Promise((resolve, reject) => {
    console.log("initialized");

    function construct_my_address_space(server) {

      const addressSpace = server.engine.addressSpace;
      const namespace = addressSpace.getOwnNamespace();

      // declare a new object
      const device = namespace.addObject({
        organizedBy: addressSpace.rootFolder.objects,
        browseName: "MyDevice"
      });

      // add some variables
      // add a variable named MyVariable1 to the newly created folder "MyDevice"
      let variable1 = 1;

      // emulate variable1 changing every 500 ms
      setInterval(function () {
        variable1 += 1;
      }, 500);

      namespace.addVariable({
        componentOf: device,
        browseName: "MyVariable1",
        dataType: "Double",
        value: {
          get: function () {
            return new opcua.Variant({
              dataType: opcua.DataType.Double,
              value: variable1
            });
          }
        }
      });

      // add a variable named MyVariable2 to the newly created folder "MyDevice"
      let variable2 = 10.0;

      namespace.addVariable({

        componentOf: device,

        nodeId: "ns=1;b=1020FFAA", // some opaque NodeId in namespace 4

        browseName: "MyVariable2",

        dataType: "Double",

        value: {
          get: function () {
            return new opcua.Variant({
              dataType: opcua.DataType.Double,
              value: variable2
            });
          },
          set: function (variant) {
            variable2 = parseFloat(variant.value);
            return opcua.StatusCodes.Good;
          }
        }
      });
      const os = require("os");
      /**
       * returns the percentage of free memory on the running machine
       * @return {double}
       */
      function available_memory() {
        // var value = process.memoryUsage().heapUsed / 1000000;
        const percentageMemUsed = os.freemem() / os.totalmem() * 100.0;
        return percentageMemUsed;
      }
      namespace.addVariable({

        componentOf: device,

        nodeId: "s=free_memory", // a string nodeID
        browseName: "FreeMemory",
        dataType: "Double",
        value: {
          get: function () {
            return new opcua.Variant({
              dataType: opcua.DataType.Double,
              value: available_memory()
            });
          }
        }
      });
    }
    construct_my_address_space(server);
    server.start(function () {
      console.log("Server is now listening... (press CTRL+C to stop)");
      console.log("Port ", server.endpoints[0].port);
      const endpointUrl = server.endpoints[0].endpointDescriptions()[0].endpointUrl;
      console.log("The primary server endpoint url is: ", endpointUrl);
      resolve(0);
    });
  })
}

async function StartServer() {
  return new Promise(async (resolve, reject) => {
    await server.initialize();
    await post_initialize();
    resolve(0);
  });
}

async function StopServer() {
  return new Promise(async(resolve, reject) => {
    console.log('Server about to shut down.');
    await server.shutdown();
    console.log('Server shut down. Why is the node process not exiting?');
    resolve(0);
  })
}

(async () => {
  await StartServer();
  await StopServer();
})().catch(e => {
  console.log(e);
  // Deal with the fact the chain failed
});


// module.exports.StartServer = StartServer;
// module.exports.StopServer = StopServer;