VisualBasic.Vsa.VsaEngine.Compile throw NullReferenceException

I'm writting a C# program use Microsoft.VisualBasic.Vsa.VsaEngine , the
program may change script text some times , when change script text , I call
some method by follow
VsaEngine.Reset
Romve all item from VsaEngine.Items
Readd Refrence item to Items
Readd Globalobject item to items
Readd Code item to items

after that , when I call VsaEngine.Compile , some times( not all time )
application throw NullReferenceException, why ?
when Compile successfly , the application run very well .( pool English )

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xdesigner