Fix for level export. (#656)

Reproduced the issue where a newly exported level would not display anything when loaded in the editor, verified this change fixes it. Note that this is all related to the legacy level system, not the new prefab level system.

Code/Sandbox/Editor/GameExporter.cpp

@@ -295,6 +295,17 @@ void CGameExporter::ExportLevelData(const QString& path, bool /*bExportMission*/
     CCryMemFile fileAction;
     fileAction.Write(xmlDataAction.c_str(), xmlDataAction.length());
     m_levelPak.m_pakFile.UpdateFile(levelDataActionFile.toUtf8().data(), fileAction);
+
+    AZStd::vector<char> entitySaveBuffer;
+    AZ::IO::ByteContainerStream<AZStd::vector<char> > entitySaveStream(&entitySaveBuffer);
+    bool savedEntities = false;
+    EBUS_EVENT_RESULT(savedEntities, AzToolsFramework::EditorEntityContextRequestBus, SaveToStreamForGame, entitySaveStream, AZ::DataStream::ST_BINARY);
+    if (savedEntities)
+    {
+        QString entitiesFile;
+        entitiesFile = QStringLiteral("%1%2.entities_xml").arg(path, "Mission0");
+        m_levelPak.m_pakFile.UpdateFile(entitiesFile.toUtf8().data(), entitySaveBuffer.begin(), entitySaveBuffer.size());
+    }
 }
 
 //////////////////////////////////////////////////////////////////////////