fix: 503 error fixed when navigating to homepage (#21971)

TL;DR 503 error fixed when navigating to homepage while the editor is
loading.

- `RESET_EDITOR_REQUEST` is an action that removes `currentApplication`
from the `applicationsReducer`.
- When starting the editor, appEngine saga has an action `completeChore`
which reads the `currentApplication` object.
- Now, when we go back to homepage (while the editor is still
initializing)
- `RESET_EDITOR_REQUEST` is dispatched (so `currentApplication` is
cleared)
- the appEngine still runs in the background and tries to read
`currentApplication` throwing an error.

Now we make sure appEngine is cancelled whenever `RESET_EDITOR_REQUEST`
is triggered.

A follow up task to investigate how errors are thrown in app editor
saga: #21972

Fixes #21915


## Media
Before:
https://www.loom.com/share/eae1eeeec04f4b35a9d3be6ea8177b0a

After:
https://www.loom.com/share/09c328f81d8344569c0ff0b7a3d97713

app/client/src/sagas/InitSagas.ts

@@ -148,10 +148,22 @@ function* updateURLSaga(action: ReduxURLChangeAction) {
   });
 }
 
+function* appEngineSaga(action: ReduxAction<AppEnginePayload>) {
+  yield race({
+    task: call(startAppEngine, action),
+    cancel: take(ReduxActionTypes.RESET_EDITOR_REQUEST),
+  });
+}
+
 export default function* watchInitSagas() {
   yield all([
-    takeLatest(ReduxActionTypes.INITIALIZE_EDITOR, startAppEngine),
-    takeLatest(ReduxActionTypes.INITIALIZE_PAGE_VIEWER, startAppEngine),
+    takeLatest(
+      [
+        ReduxActionTypes.INITIALIZE_EDITOR,
+        ReduxActionTypes.INITIALIZE_PAGE_VIEWER,
+      ],
+      appEngineSaga,
+    ),
     takeLatest(ReduxActionTypes.RESET_EDITOR_REQUEST, resetEditorSaga),
     takeEvery(URL_CHANGE_ACTIONS, updateURLSaga),
   ]);