Fixed two bugs

1- Sometimes the scroll view freezes; the error message says `+[CATransaction synchronize] called within transaction.`
 The reason was, previous code could cause something like this to happen
``` 
[CATransaction begin];
...
[CATransaction begin];
...
[CATransaction commit];
...
[CATransaction commit];
```

2- Fixed the error of trying to add a nil object to NSMutableSet *_visibleAnnotations
So I added nil checks before adding objects to the mutable set. But there must be a deeper problem that causes this to happen; this is a simple dirty fix. 




I haven't really changed a lot of code; but I think I had CLang Auto Formatting turned on, so it looks like I've changed a lot of code.