Use weak storage for the event monitor object.

Rebel/RBLPopover.m

@@ -49,7 +49,7 @@ static NSTimeInterval const RBLPopoverDefaultFadeDuration = 0.3;
 // The identifier for the event monitor we are using to watch for mouse clicks
 // outisde of the popover.
 // We are not responsible for it's memory management.
-@property (nonatomic, unsafe_unretained) id transientEventMonitor;
+@property (nonatomic, weak) id transientEventMonitor;
 
 // Whether the popover is currently animating, either in or out.
 @property (nonatomic, getter = isAnimating) BOOL animating;