Request: Make scrollNavigationBar nullable

[Errortype520]

I think scrollingnavigationbar should be nullable for use with Swift.

` @interface UINavigationController (GTScrollNavigationBarAdditions)

@property(strong, nonatomic, readonly, nullable) GTScrollNavigationBar *scrollNavigationBar;

@end

@implementation UINavigationController (GTScrollNavigationBarAdditions)

@dynamic scrollNavigationBar;

• (nullable GTScrollNavigationBar)scrollNavigationBar { return [self.navigationBar isKindOfClass:[GTScrollNavigationBar class]] ? (GTScrollNavigationBar)self.navigationBar : nil; }

@end `

Swift example:

self.navigationController?.scrollNavigationBar?.scrollView = self.tableView