apple: fix timing issue crash in executeApplicationScript (#134)

* fix timing issue with _reactInstance going away between null check and call

* Move comment

React/CxxBridge/RCTCxxBridge.mm

@@ -1302,19 +1302,21 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
     [[NSNotificationCenter defaultCenter]
       postNotificationName:RCTJavaScriptWillStartExecutingNotification
       object:self->_parentBridge userInfo:@{@"bridge": self}];
+    // TODO(OSS Candidate ISS#2710739) - use local reactInstance, instance can be null'd while constructing NSDataBigString
+    auto reactInstance = self->_reactInstance;
     if (isRAMBundle(script)) {
       [self->_performanceLogger markStartForTag:RCTPLRAMBundleLoad];
       auto ramBundle = std::make_unique<JSIndexedRAMBundle>(sourceUrlStr.UTF8String);
       std::unique_ptr<const JSBigString> scriptStr = ramBundle->getStartupCode();
       [self->_performanceLogger markStopForTag:RCTPLRAMBundleLoad];
       [self->_performanceLogger setValue:scriptStr->size() forTag:RCTPLRAMStartupCodeSize];
-      if (self->_reactInstance) {
+      if (reactInstance) {
         auto registry = RAMBundleRegistry::multipleBundlesRegistry(std::move(ramBundle), JSIndexedRAMBundle::buildFactory());
-        self->_reactInstance->loadRAMBundle(std::move(registry), std::move(scriptStr),
+        reactInstance->loadRAMBundle(std::move(registry), std::move(scriptStr),
                                             sourceUrlStr.UTF8String, !async);
       }
-    } else if (self->_reactInstance) {
+    } else if (reactInstance) {
-      self->_reactInstance->loadScriptFromString(std::make_unique<NSDataBigString>(script), 0,
+      reactInstance->loadScriptFromString(std::make_unique<NSDataBigString>(script), 0,
                                                  sourceUrlStr.UTF8String, !async, ""); // TODO(OSS Candidate ISS#2710739)
     } else {
       std::string methodName = async ? "loadApplicationScript" : "loadApplicationScriptSync";