Unit testing a private property in view controller loaded from the storyboard

I have a view controller called SubMenuViewController and a label in it called selectionLabel. The view controller is present in the storyboard and I am writing unit tests for testing SubMenuViewController.

I have a method:

- (void)drawerItemSelectedWithIndex:(NSInteger)selectionIndex 
{
    if(selectionIndex == 0)
    {
        self.selectionLabel.text = @"All";
    }
    else
    {
        self.selectionLabel.text = @"Test";
    }
}

I want to write unit test cases for this. I have created an extension.

@interface SubMenuViewControllerTests : XCTestCase
@property(nonatomic)SubMenuViewController *subMenuViewController;

@end

@interface SubMenuViewController()
@property(nonatomic)IBOutlet UILabel *selectionLabel;
@end

And This is the test method I have written:

-(void)testCCKFNavDrawerSelectionWithIndex
{
    [self.subMenuViewController drawerItemSelectedWithIndex:0];
    XCTAssert([self.subMenuViewController.selectionLabel.text isEqualToString:@"All" ]);
}

Is this the right way to test this method, as I am getting value of self.subMenuViewController.selectionLabel as nil.