Minor Issues

I had an issue when retaining a instance of the RNFrostedSidebar as a property, the item views would not animate after the first show call. I also realized that rn_frostedSidebar never got cleared. I dont know if this was intentional but it seems that when the sidebar is dismissed it should be set to nil. 

While fixing that issue I came across some other minor issues that were causing me warnings but not really affecting the code. 
It seems that the topPadding is more appropriate to use for the height calculation just in case they were to change.