fix: 修复切换模型时报 undefined 的问题 (#775)

src/utils/live2d.ts

@@ -73,7 +73,11 @@ class Live2d {
   }
 
   public destroy() {
+    if (!this.model) return
+
     this.model?.destroy()
+
+    this.model = null
   }
 
   public resizeModel(modelSize: ModelSize) {