Why are these three cases markes with possible NullReferenceException

Why are these three cases markes with possible NullReferenceException:

using System;  

namespace WhyPossibleNullReference 

{

      class SampleClass

      {

           private static readonly object TestObject1 = new object();

           private readonly object testObject2 = new object();

           private readonly object testObject3;

          public SampleClass(object testObject3)

           {

                if (testObject3 == null)

                     throw new ArgumentNullException("testObject3");

                this.testObject3 = testObject3;

           }

          public void DoSomething()

           {

                // case 1:

               // why is the object here marked with "possible NullReferenceException"?

                // the static constructor should be called before this method and thus the field is initialized

                Console.WriteLine(TestObject1.ToString());

               // case 2:

               // why is the object here marked with "possible NullReferenceException"?

                // the field is set before the constructor is called thus the field is initialized

                Console.WriteLine(testObject2.ToString());

               // case 3:

               // why is the object here marked with "possible NullReferenceException"?

                // the field is set in the constructor and a null check made sure that it is not set to null

                Console.WriteLine(testObject3.ToString());

           }

      }

 }