Fix issue #2 Unable to package with the plugin enabled

When packaging a project, the Source Control Plugin is triggered, and try to register a Menu Extension.
But the LevelEditor module is unavailable at this step, which raises an assertion.

Now uses FModuleManager::GetModulePtr() and test the returned pointer instead of FModuleManager::LoadModuleChecked() that was causing the assertion.

Source/PlasticSourceControl/Private/PlasticSourceControlMenu.cpp

@@ -44,20 +44,26 @@ void FPlasticSourceControlMenu::Register()
 
 	// Register the extension with the level editor
 	{
-		FLevelEditorModule::FLevelEditorMenuExtender ViewMenuExtender = FLevelEditorModule::FLevelEditorMenuExtender::CreateRaw(this, &FPlasticSourceControlMenu::OnExtendLevelEditorViewMenu);
-		FLevelEditorModule& LevelEditorModule = FModuleManager::LoadModuleChecked<FLevelEditorModule>("LevelEditor");
-		auto& MenuExtenders = LevelEditorModule.GetAllLevelEditorToolbarSourceControlMenuExtenders();
-		MenuExtenders.Add(ViewMenuExtender);
-		ViewMenuExtenderHandle = MenuExtenders.Last().GetHandle();
+		FLevelEditorModule* LevelEditorModule = FModuleManager::GetModulePtr<FLevelEditorModule>("LevelEditor");
+		if (LevelEditorModule)
+		{
+			FLevelEditorModule::FLevelEditorMenuExtender ViewMenuExtender = FLevelEditorModule::FLevelEditorMenuExtender::CreateRaw(this, &FPlasticSourceControlMenu::OnExtendLevelEditorViewMenu);
+			auto& MenuExtenders = LevelEditorModule->GetAllLevelEditorToolbarSourceControlMenuExtenders();
+			MenuExtenders.Add(ViewMenuExtender);
+			ViewMenuExtenderHandle = MenuExtenders.Last().GetHandle();
+		}
 	}
 }
 
 void FPlasticSourceControlMenu::Unregister()
 {
 	// Unregister the level editor extensions
 	{
-		FLevelEditorModule& LevelEditorModule = FModuleManager::LoadModuleChecked<FLevelEditorModule>("LevelEditor");
-		LevelEditorModule.GetAllLevelEditorToolbarSourceControlMenuExtenders().RemoveAll([=](const FLevelEditorModule::FLevelEditorMenuExtender& Extender) { return Extender.GetHandle() == ViewMenuExtenderHandle; });
+		FLevelEditorModule* LevelEditorModule = FModuleManager::GetModulePtr<FLevelEditorModule>("LevelEditor");
+		if (LevelEditorModule)
+		{
+			LevelEditorModule->GetAllLevelEditorToolbarSourceControlMenuExtenders().RemoveAll([=](const FLevelEditorModule::FLevelEditorMenuExtender& Extender) { return Extender.GetHandle() == ViewMenuExtenderHandle; });
+		}
 	}
 
 	// This function may be called during shutdown to clean up your module.  For modules that support dynamic reloading,