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.