Fix retaining self in block in LogBox impl

Summary:
Logbox has a retain cycle (see linked task for my deeper investigation).

This diff doesn't fix the retain cycle, but it's just good practice to not retain self strongly in blocks.

Changelog: [iOS][Internal] Fix retaining self in block in LogBox implementation

Reviewed By: shergin

Differential Revision: D20630693

fbshipit-source-id: cf399495e9bcd1917932fcc0e9c9d2d2a32bf6f0

React/CoreModules/RCTLogBox.mm

@@ -91,11 +91,16 @@ RCT_EXPORT_MODULE()
 RCT_EXPORT_METHOD(show)
 {
   if (RCTRedBoxGetEnabled()) {
+    __weak RCTLogBox *weakSelf = self;
     dispatch_async(dispatch_get_main_queue(), ^{
-      if (!self->_view) {
-        self->_view = [[RCTLogBoxView alloc] initWithFrame:[UIScreen mainScreen].bounds bridge:self->_bridge];
+      __strong RCTLogBox *strongSelf =  weakSelf;
+      if (!strongSelf) {
+        return;
       }
-      [self->_view show];
+      if (!strongSelf->_view) {
+        strongSelf->_view = [[RCTLogBoxView alloc] initWithFrame:[UIScreen mainScreen].bounds bridge:self->_bridge];
+      }
+      [strongSelf->_view show];
     });
   }
 }
@@ -103,8 +108,13 @@ RCT_EXPORT_METHOD(show)
 RCT_EXPORT_METHOD(hide)
 {
   if (RCTRedBoxGetEnabled()) {
+    __weak RCTLogBox *weakSelf = self;
     dispatch_async(dispatch_get_main_queue(), ^{
-      self->_view = nil;
+      __strong RCTLogBox *strongSelf =  weakSelf;
+      if (!strongSelf) {
+        return;
+      }
+      strongSelf->_view = nil;
     });
   }
 }