Fix spine crash when exiting game. (#18506)

It's a bug after merging #18467

Author: bofeng-song

native/cocos/editor-support/spine-creator-support/SkeletonCacheAnimation.cpp

@@ -85,6 +85,7 @@ SkeletonCacheAnimation::~SkeletonCacheAnimation() {
     for (auto &item : _materialCaches) {
         CC_SAFE_DELETE(item.second);
     }
+    _entity = nullptr;
     stopSchedule();
 }
 

native/cocos/editor-support/spine-creator-support/SkeletonRenderer.cpp

@@ -174,6 +174,7 @@ SkeletonRenderer::~SkeletonRenderer() {
         CC_SAFE_DELETE(item.second);
     }
 
+    _entity = nullptr;
     stopSchedule();
 }