Added nil check to BottomViewController transition

Checking for nil value of fromVC and toVC for tabBarController:animationControllerForTransitionFromViewController

Sources/BottomNavigationController.swift

@@ -130,6 +130,11 @@ public class BottomNavigationController : UITabBarController, UITabBarController
 	
 	/// Handles transitions when tabBarItems are pressed.
 	public func tabBarController(tabBarController: UITabBarController, animationControllerForTransitionFromViewController fromVC: UIViewController, toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
+		let fromVC: UIViewController? = fromVC
+	    	let toVC: UIViewController? = toVC
+	    	if nill == fromVC || nil == toVC {
+	      		return nil
+	    	}
 		return .Fade == transitionAnimation ? BottomNavigationFadeAnimatedTransitioning() : nil
 	}