Streaming level keeps loading state and can't be loaded or unloaded!

I’ve tried the methods below,but they make no effect.How can i solve it?
Is there a force unload function?
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When you load the level, you get a reference. Try using that to unload the level?

The ULevelStreaming reference?Or the ULevel reference?

Sorry, I mean this reference

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……It keeps loading state,so the reference will be null.