[material-ui][Unstable_TrapFocus] Fix getTabbable function return type (#42237)

Co-authored-by: ZeeshanTamboli <zeeshan.tamboli@gmail.com>

Author: KalmarLorand

packages/mui-material/src/Unstable_TrapFocus/FocusTrap.tsx

@@ -270,7 +270,7 @@ function FocusTrap(props: FocusTrapProps): React.JSX.Element {
         return;
       }
 
-      let tabbable: ReadonlyArray<string> | HTMLElement[] = [];
+      let tabbable: ReadonlyArray<HTMLElement> = [];
       if (
         doc.activeElement === sentinelStart.current ||
         doc.activeElement === sentinelEnd.current

packages/mui-material/src/Unstable_TrapFocus/FocusTrap.types.ts

@@ -10,7 +10,7 @@ export interface FocusTrapProps {
    * For instance, you can provide the "tabbable" npm dependency.
    * @param {HTMLElement} root
    */
-  getTabbable?: (root: HTMLElement) => ReadonlyArray<string>;
+  getTabbable?: (root: HTMLElement) => ReadonlyArray<HTMLElement>;
   /**
    * This prop extends the `open` prop.
    * It allows to toggle the open state without having to wait for a rerender when changing the `open` prop.