Use order as key when registering block components rather than ID

packages/client/src/components/Block.svelte

@@ -14,7 +14,7 @@
     }
     // Remove existing instance of this component in case props changed
     structureLookupMap[parentId] = structureLookupMap[parentId].filter(
-      blockComponent => blockComponent.instance._id !== id
+      blockComponent => blockComponent.order !== order
     )
     // Add new instance of this component
     structureLookupMap[parentId].push({ order, instance })

packages/client/src/components/BlockComponent.svelte

@@ -39,7 +39,7 @@
   // ejected later
   $: {
     if ($builderStore.inBuilder) {
-      block.registerComponent(id, order, $component?.id, instance)
+      block.registerComponent(id, order ?? 0, $component?.id, instance)
     }
   }
 </script>