Fix console errors in some cases

src/components/cluster.vue

@@ -103,11 +103,14 @@ export default buildComponent({
   },
   beforeUnmount() {
     /* Performance optimization when destroying a large number of markers */
+    if (this.$children && this.$children.length) {
       this.$children.forEach((marker) => {
         if (marker.$clusterObject === this.$clusterObject) {
           marker.$clusterObject = null
         }
       })
+    }
+
 
     if (this.$clusterObject) {
       this.$clusterObject.clearMarkers()