How to launch click-once application in Win App driver?

As mentioned here, you need to check the driver is null.

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using Microsoft.VisualStudio.TestTools.UnitTesting;
using OpenQA.Selenium.Appium.Windows;
using OpenQA.Selenium.Remote;
using System;

namespace CalculatorTest
{
    public class CalculatorSession
    {
        // Note: append /wd/hub to the URL if you're directing the test at Appium
        private const string WindowsApplicationDriverUrl = "http://127.0.0.1:4723";
        private const string CalculatorAppId = "Microsoft.WindowsCalculator_8wekyb3d8bbwe!App";

        protected static WindowsDriver<WindowsElement> session;

        public static void Setup(TestContext context)
        {
            // Launch Calculator application if it is not yet launched
            if (session == null)
            {
                // Create a new session to bring up an instance of the Calculator application
                // Note: Multiple calculator windows (instances) share the same process Id
                DesiredCapabilities appCapabilities = new DesiredCapabilities();
                appCapabilities.SetCapability("app", CalculatorAppId);
                appCapabilities.SetCapability("deviceName", "WindowsPC");
                session = new WindowsDriver<WindowsElement>(new Uri(WindowsApplicationDriverUrl), appCapabilities);
                Assert.IsNotNull(session);

                // Set implicit timeout to 1.5 seconds to make element search to retry every 500 ms for at most three times
                session.Manage().Timeouts().ImplicitWait = TimeSpan.FromSeconds(1.5);
            }
        }

        public static void TearDown()
        {
            // Close the application and delete the session
            if (session != null)
            { 
                session.Quit();
                session = null;
            }
        }
    }
}