Test Runner gives me "MissingReferenceException"

Hi guys
I created a project from scratch and added 2 classes to run under test runner. Test runner was working but editor gave me some errors i didn't understand.

Calculator.cs

Code (CSharp):

1.  
2. public class Calculator
3. {
4.     public double Add(double a, double b) => a + b;
5.     public double Subtract(double a, double b) => a - b;
6.     public double Multiply(double a, double b) => a * b;
7.     public double Divide(double a, double b) => a / b;
8. }
9.  

Send.cs

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class Send
6. {
7.     public string SendHello()
8.     {
9.        
10.         return "Hello";
11.     }
12.  
13. }
14.  

CalculatorTest.cs

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using NUnit.Framework;
4. using UnityEngine;
5. using UnityEngine.TestTools;
6.  
7. namespace Tests
8. {
9.     public class CalculatorTest
10.     {
11.         [Test]
12.         public void Add_SimpleValuesShouldCalculate()
13.         {
14.             double expected = 2 + 3;
15.             double actual = new Calculator().Add(2, 3);
16.             //Assert.Equals(expected, actual);
17.         }
18.         [TestCase(2,3, ExpectedResult = 5, Author = "Author")]
19.         public double Add_SimpleValuesShouldCalculate(double a, double b)
20.         {
21.             double actual = new Calculator().Add(a, b);
22.             return actual;
23.         }
24.  
25.         [TestCase(2, 3, 4, ExpectedResult = 9, Author = "Author")]
26.         public double Add_SimpleValuesShouldCalculate(double a, double b, double c)
27.         {
28.             double actual = new Calculator().Add(a, b);
29.             return actual + c;
30.         }
31.         //[TestCase(ExpectedResult = "Hello")]
32.         //[UnityPlatform(include = new[] { RuntimePlatform.WindowsEditor })]
33.         //public string SayHello()
34.         //{
35.         //    return new Send().SendHello();
36.         //}
37.     }
38. }
39.