How to fix this System.NullReferenceException in Test class? [duplicate]

Usually, I know how to fix this kind of Exception. However, this time, due to it being because of working with Test classes, I am unsure how to pass the correct parameter.

The method that I am testing is Update() inside PhaseController.

    public IActionResult Update(int id)
    {
        string connectionString = Configuration["ConnectionStrings:DefaultConnection"];

        Phase phase = new Phase();
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            string sql = $"Select * From Phases Where Id='{id}'";
            SqlCommand command = new SqlCommand(sql, connection);

            connection.Open();

            using (SqlDataReader dataReader = command.ExecuteReader())
            {
                while (dataReader.Read())
                {
                    phase.Id = Convert.ToInt32(dataReader["Id"]);
                    phase.Name = Convert.ToString(dataReader["Name"]);
                }
            }

            connection.Close();
        }
        return View(phase);
    }

And my PhaseControllerTest class is this:

public class PhaseControllerTest
{
    public IConfiguration Configuration { get; }

    [Fact]
    public void TestCreateView()
    {
        var controller = new PhaseController(Configuration);
        var result = controller.Create() as ViewResult;
        Assert.Contains("Create", result.ViewName);
    }

    [Fact]
    public void TestUpdateView()
    {
        Phase phase = new Phase();
        phase.Id = 6009;
        phase.Name = "Main Phase";
        phase.ReleaseId = 3008;
        int id = 6009;
        var controller = new PhaseController(Configuration);
        var result = controller.Update(id) as ViewResult;
        Assert.True(phase.Id == ((Phase) result.Model).Id);
    }
}

The error I am getting is on this line:

string connectionString = Configuration["ConnectionStrings:DefaultConnection"];

Saying:

System.NullReferenceException: 'Object reference not set to an instance of an object.' Intersection.Controllers.PhaseController.Configuration.get returned null.

How can I insert the Configuration in the TestUpdateView()? Preferably, without actually having to write down the whole connection string.