Return early if tab or panel are undefined

Author: camertron

src/tab-container-element.ts

@@ -362,6 +362,9 @@ export class TabContainerElement extends HTMLElement {
     const selectedTab = tabs[index]
     const selectedPanel = panels[index]
 
+    if (!selectedTab) return
+    if (!selectedPanel) return
+
     if (this.#setupComplete) {
       const cancelled = !this.dispatchEvent(
         new TabContainerChangeEvent('tab-container-change', {