chore: TypeError when deleting in zen (@byseif21) (monkeytypegame#7282)

* fix error from race condition `Cannot read properties of null (reading
'remove')` when deletion in zen mode, added null check.

The active word could already be removed when the debounced update runs,
which caused a null error

---------

Co-authored-by: Jack <[email protected]>

Author: byseif21

frontend/src/ts/test/test-ui.ts

@@ -133,18 +133,17 @@ export function updateActiveElement(
 
     let previousActiveWordTop: number | null = null;
     if (initial === undefined) {
-      const previousActiveWord = wordsEl.querySelector(
-        ".active",
-      ) as HTMLElement;
-      if (direction === "forward") {
-        previousActiveWord.classList.add("typed");
-      } else if (direction === "back") {
-        if (Config.mode === "zen") {
-          previousActiveWord.remove();
+      const previousActiveWord = wordsEl.querySelector<HTMLElement>(".active");
+      // in zen mode, because of the animation frame, previousActiveWord will be removed at this point, so check for null
+      if (previousActiveWord !== null) {
+        if (direction === "forward") {
+          previousActiveWord.classList.add("typed");
+        } else if (direction === "back") {
+          //
         }
+        previousActiveWord.classList.remove("active");
+        previousActiveWordTop = previousActiveWord.offsetTop;
       }
-      previousActiveWord.classList.remove("active");
-      previousActiveWordTop = previousActiveWord.offsetTop;
     }
 
     const newActiveWord = getActiveWordElement();