Prevent possible race condition in NotifyAppControl (#54)

Author: swift-kim

flutter/shell/platform/tizen/channels/app_control.h

@@ -120,8 +120,13 @@ class AppControlManager {
     return instance;
   }
 
-  void Insert(std::unique_ptr<AppControl> app_control) {
-    map_.insert({app_control->id(), std::move(app_control)});
+  // Returns a pointer to the inserted element if successful, otherwise nullptr.
+  AppControl* Insert(std::unique_ptr<AppControl> app_control) {
+    auto iter = map_.emplace(app_control->id(), std::move(app_control));
+    if (iter.second) {
+      return iter.first->second.get();
+    }
+    return nullptr;
   }
 
   void Remove(int32_t id) { map_.erase(id); }

flutter/shell/platform/tizen/channels/app_control_channel.cc

@@ -58,13 +58,18 @@ void AppControlChannel::NotifyAppControl(void* handle) {
     FT_LOG(Error) << "Could not create an instance of AppControl.";
     return;
   }
+  AppControl* app_control_ptr =
+      AppControlManager::GetInstance().Insert(std::move(app_control));
+  if (!app_control_ptr) {
+    FT_LOG(Error) << "The handle already exists.";
+    return;
+  }
   if (event_sink_) {
-    SendAppControlEvent(app_control.get());
+    SendAppControlEvent(app_control_ptr);
   } else {
     FT_LOG(Info) << "No event channel has been set up.";
-    queue_.push(app_control.get());
+    queue_.push(app_control_ptr);
   }
-  AppControlManager::GetInstance().Insert(std::move(app_control));
 }
 
 void AppControlChannel::HandleMethodCall(