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
Added nil checks for objects to be added to that mutable set




I haven't really changed a lot of code; but I think I turned on CLang Auto Formatting so it's going crazy