fix https://github.com/sveltejs/svelte/issues/2086: detach error when node is not attached to DOM

src/runtime/internal/dom.ts

@@ -7,7 +7,9 @@ export function insert(target: Node, node: Node, anchor?: Node) {
 }
 
 export function detach(node: Node) {
-	node.parentNode.removeChild(node);
+	if (node.parentNode) {
+		node.parentNode.removeChild(node);
+	}
 }
 
 export function detach_between(before: Node, after: Node) {