IPlacementModeModule::Get() crash

every time i call IPlacementModeModule::Get() , the engine crashs.
I dont know why.
can not load this module from my plugin.

#include "test.h"
    #include "IPlacementModeModule.h"
    #define LOCTEXT_NAMESPACE "FtestModule"
    void FtestModule::StartupModule()
    	// This code will execute after your module is loaded into memory; the exact timing is specified in the .uplugin file per-module
    	IPlacementModeModule& PlacementModeModule = IPlacementModeModule::Get();
    void FtestModule::ShutdownModule()
    	// This function may be called during shutdown to clean up your module.  For modules that support dynamic reloading,
    	// we call this function before unloading the module.
    IMPLEMENT_MODULE(FtestModule, test)