Class1.cs

using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Firefox;

namespace sandbox
{
    [TestFixture]
    class Class1
    {
        IWebDriver Driver;

        [SetUp]
        public void SetUp()
        {
            Driver = new FirefoxDriver();
        }

        [TearDown]
        public void TearDown()
        {
            Driver.Quit();
        }

        [Test]
        public void ValidAccount()
        {
            Driver.Navigate().GoToUrl("http://the-internet.herokuapp.com/login");
            Driver.FindElement(By.Id("username")).SendKeys("tomsmith");
            Driver.FindElement(By.Id("password")).SendKeys("SuperSecretPassword!");
            Driver.FindElement(By.CssSelector("button")).Click();
        }
    }
}

sandbox.proj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netcoreapp2.0</TargetFramework>

    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="nunit" Version="3.11.0" />
    <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.0.1" />
    <PackageReference Include="Selenium.Support" Version="4.0.0-alpha01" />
    <PackageReference Include="Selenium.WebDriver" Version="4.0.0-alpha01" />
  </ItemGroup>

</Project>

stacktrace

  Error Message:
   System.IO.FileNotFoundException : Could not load file or assembly 'Newtonsoft.Json, Version=12.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'. The system cannot find the file specified.

TearDown : System.NullReferenceException : Object reference not set to an instance of an object.
  Stack Trace:
     at OpenQA.Selenium.Remote.Command.get_ParametersAsJsonString()
   at OpenQA.Selenium.Remote.HttpCommandExecutor.HttpRequestInfo..ctor(Uri serverUri, Command commandToExecute, CommandInfo commandInfo)
   at OpenQA.Selenium.Remote.HttpCommandExecutor.Execute(Command commandToExecute)
   at OpenQA.Selenium.Remote.DriverServiceCommandExecutor.Execute(Command commandToExecute)
   at OpenQA.Selenium.Remote.RemoteWebDriver.Execute(String driverCommandToExecute, Dictionary`2 parameters)
   at OpenQA.Selenium.Remote.RemoteWebDriver.StartSession(ICapabilities desiredCapabilities)
   at OpenQA.Selenium.Remote.RemoteWebDriver..ctor(ICommandExecutor commandExecutor, ICapabilities desiredCapabilities)
   at OpenQA.Selenium.Firefox.FirefoxDriver..ctor(FirefoxDriverService service, FirefoxOptions options, TimeSpan commandTimeout)
   at sandbox.Class1.SetUp() in /Users/tourdedave/sandbox/Class1.cs:line 15
--TearDown
   at sandbox.Class1.TearDown() in /Users/tourdedave/sandbox/Class1.cs:line 21