make sure to clean up

src/Umbraco.Web.UI.Client/src/libs/extension-api/controller/base-extension-initializer.controller.ts

@@ -176,6 +176,12 @@ export abstract class UmbBaseExtensionInitializer<
 			configsOfThisType.map((conditionConfig) => this.#createConditionController(conditionManifest, conditionConfig)),
 		);
 
+		// If we got destroyed in the mean time, then we don't need to continue:
+		if (!this.#extensionRegistry) {
+			newConditionControllers.forEach((controller) => controller?.destroy());
+			return;
+		}
+
 		const oldLength = this.#conditionControllers.length;
 
 		newConditionControllers

src/Umbraco.Web.UI.Client/src/packages/core/components/extension-with-api-slot/extension-with-api-slot.element.ts

@@ -153,6 +153,7 @@ export class UmbExtensionWithApiSlotElement extends UmbLitElement {
 	}
 
 	#observeExtensions(): void {
+		// We want to be attached before we start observing extensions, cause first at this point we know that we got the right properties. [NL]
 		if (!this.#attached) return;
 		this.#extensionsController?.destroy();
 		if (this.#type) {