Test for a UIViewController fails after refactor in Swift

I'm having different results in trying to test a ViewController in Swift.

This first code pass the test.

@testable import VideoAudioExtractor
import XCTest

class SecondViewControllerTest: XCTestCase {

let storyBoardName = "Main"
let viewControllerIdentifier = "SecondViewController"

override func setUp() {
    super.setUp()
    // Put setup code here. This method is called before the invocation of each test method in the class.
}

func testSelectAudioButtonIsConnected () {
    let sut = UIStoryboard(name: storyBoardName, bundle: nil).instantiateViewControllerWithIdentifier("SecondViewController") as! SecondViewController
    let dummy = sut.view
    if let unpwarppedOptional = sut.selectAudioButton {
        XCTAssertEqual(unpwarppedOptional,sut.selectAudioButton, "correct value")
    }
    else {
        XCTFail("Value isn't set")
    }
}

override func tearDown() {
    // Put teardown code here. This method is called after the invocation of each test method in the class.
    super.tearDown()
}

}

If I refactor the test and I move the creation of the view controller to an instance variable the test fails in Line

@testable import VideoAudioExtractor
import XCTest

class SecondViewControllerTest: XCTestCase {

let storyBoardName = "Main"
let viewControllerIdentifier = "SecondViewController"
var sut : SecondViewController {
    return UIStoryboard(name: storyBoardName, bundle: nil).instantiateViewControllerWithIdentifier("SecondViewController") as! SecondViewController
}

override func setUp() {
    super.setUp()
    // Put setup code here. This method is called before the invocation of each test method in the class.
}

func testSelectAudioButtonIsConnected () {

    let dummy = sut.view
    if let unpwarppedOptional = sut.selectAudioButton {
        XCTAssertEqual(unpwarppedOptional,sut.selectAudioButton, "correct value")
    }
    else {
        XCTFail("Value isn't set")
    }
}

override func tearDown() {
    // Put teardown code here. This method is called after the invocation of each test method in the class.
    super.tearDown()
}

}