fix: additional check for component on destroy (#11488)

fixes #10454

.changeset/olive-apples-lick.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check if svelte component exists on custom element destroy

packages/svelte/src/internal/client/dom/elements/custom-element.js

@@ -193,7 +193,7 @@ if (typeof HTMLElement === 'function') {
 			this.$$cn = false;
 			// In a microtask, because this could be a move within the DOM
 			Promise.resolve().then(() => {
-				if (!this.$$cn) {
+				if (!this.$$cn && this.$$c) {
 					this.$$c.$destroy();
 					destroy_effect(this.$$me);
 					this.$$c = undefined;