ahaha wtf y "" != null?

thx i rly dont forgot about one :D, but can u explain to me, what the compiler wants to see from me :)
Test Failed
Expected: <string.Empty>
But was: null

wtf, y im send my code and if i click test then my code compiled and send me status true :sample test completed, but if i send finaly tests(attempt) then sample test not compilied and send me error NullReferences wtf guys
//click on button send
Test Results:
Solution
SolutionTest
SampleTest
Completed in 20.5900ms
Completed in 23.5130ms
You have passed all of the tests! :)
//click on button attempt
Test Results:
Solution
SolutionTest
Random Tests
SampleTest
System.NullReferenceException : Object reference not set to an instance of an object.
Stack Trace
at Kata.AlphabetPosition(String text)
at Solution.SolutionTest.SampleTest()
Completed in 16.0730ms
Completed in 58.1370ms
Completed in 60.9550ms