Not able to add tabbarcontroller to leftviewcontroller in swift

Thanks for the great library. But, we are facing some issue, i.e, I have added 5 viewcontrollers to leftviewcontroller. so its showing 4 rows in left side tableview. But, I am trying to add 2 cell data to navigate to tabbarcontroller for that we added like following.

```
var viewAssetMainTabBarViewController: UITabBarController!

override func viewDidLoad() {
super.viewDidLoad()

let viewAssetsTabbarController = storyboard.instantiateViewController(withIdentifier: "ViewAssetMainTabBarViewController") as! ViewAssetMainTabBarViewController

}

func changeViewController(_ menu: LeftMenu) {
switch menu {

case .assets:
self.slideMenuController()?.changeMainViewController(self.viewAssetMainTabBarViewController, close: true)

}

But, after clicking on that row in tableview its getting crashed and throwing following error in console
Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value

```

Can anyone suggest me, how to fix this?