Reproduce steps:

1. Download https://github.com/ncsoft/Unreal.js-demo.git then unzip Unreal.js-demo-master.zip
2. Open Demo.uproject using UE4.21.2.
3. Play level 'Demo' in editor then stop play.
4. Open level 'StarterContent/Maps/Advanced_Lighting' or other level.

Result:
The editor crashes.

Crash information:
Fatal error: [File:D:\Build++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\EditorServer.cpp] [Line: 1969] /Game/Demo.Demo still around while trying to load new map (Object is not currently rooted)

UE4Editor_Core!FDebug::AssertFailed() [d:\build++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:417]
UE4Editor_UnrealEd!UEditorEngine::CheckForWorldGCLeaks() [d:\build++ue4\sync\engine\source\editor\unrealed\private\editorserver.cpp:1969]
UE4Editor_UnrealEd!UEditorEngine::EditorDestroyWorld() [d:\build++ue4\sync\engine\source\editor\unrealed\private\editorserver.cpp:2105]
UE4Editor_UnrealEd!UEditorEngine::Map_Load() [d:\build++ue4\sync\engine\source\editor\unrealed\private\editorserver.cpp:2462]
UE4Editor_UnrealEd!UEditorEngine::HandleMapCommand() [d:\build++ue4\sync\engine\source\editor\unrealed\private\editorserver.cpp:6023]
UE4Editor_UnrealEd!UEditorEngine::Exec() [d:\build++ue4\sync\engine\source\editor\unrealed\private\editorserver.cpp:5500]
UE4Editor_UnrealEd!UUnrealEdEngine::Exec() [d:\build++ue4\sync\engine\source\editor\unrealed\private\unrealedsrv.cpp:691]
UE4Editor_UnrealEd!FEditorFileUtils::LoadMap() [d:\build++ue4\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2396]
UE4Editor_AssetTools!FAssetTypeActions_World::OpenAssetEditor() [d:\build++ue4\sync\engine\source\developer\assettools\private\assettypeactions\assettypeactions_world.cpp:32]
UE4Editor_UnrealEd!FAssetEditorManager::OpenEditorForAsset() [d:\build++ue4\sync\engine\source\editor\unrealed\private\toolkits\asseteditormanager.cpp:380]
UE4Editor_AssetTools!FAssetTypeActions_Base::AssetsActivated() [d:\build++ue4\sync\engine\source\developer\assettools\public\assettypeactions_base.h:57]
UE4Editor_ContentBrowser!SContentBrowser::OnAssetsActivated() [d:\build++ue4\sync\engine\source\editor\contentbrowser\private\scontentbrowser.cpp:2112]
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,SContentBrowser,0,void __cdecl(TArray<FAssetData,FDefaultAllocator> const & __ptr64,enum EAssetTypeActivationMethod::Type)>::ExecuteIfSafe() [d:\build++ue4\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:355]
UE4Editor_ContentBrowser!SAssetView::OnListMouseButtonDoubleClick() [d:\build++ue4\sync\engine\source\editor\contentbrowser\private\sassetview.cpp:4058]
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,SAssetView,0,void __cdecl(TSharedPtr<FAssetViewItem,0>)>::ExecuteIfSafe() [d:\build++ue4\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:355]
UE4Editor_ContentBrowser!TBaseDelegate<void,TSharedPtr<FAssetViewItem,0> >::ExecuteIfBound() [d:\build++ue4\sync\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:648]
UE4Editor_ContentBrowser!SListView<TSharedPtr<FAssetViewItem,0> >::Private_OnItemDoubleClicked() [d:\build++ue4\sync\engine\source\runtime\slate\public\widgets\views\slistview.h:815]
UE4Editor_ContentBrowser!STableRow<TSharedPtr<FAssetViewItem,0> >::OnMouseButtonDoubleClick() [d:\build++ue4\sync\engine\source\runtime\slate\public\widgets\views\stablerow.h:310]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FBubblePolicy,FPointerEvent,<lambda_eeb33fd1b480e3cad58a1531d90d2e14> >() [d:\build++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:270]
UE4Editor_Slate!FSlateApplication::RoutePointerDoubleClickEvent() [d:\build++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:6023]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonDoubleClickEvent() [d:\build++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:6010]
UE4Editor_Slate!FSlateApplication::OnMouseDoubleClick() [d:\build++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5980]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() [d:\build++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1835]
UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() [d:\build++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:2281]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() [d:\build++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:929]
UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() [d:\build++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:766]
user32
user32
UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() [d:\build++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsplatformapplicationmisc.cpp:130]
UE4Editor!FEngineLoop::Tick() [d:\build++ue4\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3615]
UE4Editor!GuardedMain() [d:\build++ue4\sync\engine\source\runtime\launch\private\launch.cpp:174]
UE4Editor!GuardedMainWrapper() [d:\build++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:145]
UE4Editor!WinMain() [d:\build++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:276]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283]
kernel32
ntdll

UE4 version: 4.21.2 from Epic Games Launcher.
Unreal.js version: 0.3.3 from Unreal Engine Marketplace
OS: Windows 10 64bit