fix shutdown crash on windows

[FlauschBert]

Concernig https://github.com/effekseer/EffekseerForGodot4/issues/2#issuecomment-1672130188.
I fixed the shutdown crash.

• Tested on windows with a completely empty project and the gdextension.
• Tested on windows with activated addon and no Effekseer elements.
• Tested on windows with activated addon and some Effekseer examples.

There are still dangling references after the gdextension was unloaded/ plugin removed from tree.

[FlauschBert]

Hm. I tested the Demo assigned to this project. It crashes while first loading ... so I will have to check some things out.