remove optional chaning

until CodeSandbox adds support for it in CRA apps

src/index.tsx

@@ -284,7 +284,7 @@ class App extends React.Component<{}, AppState> {
   private onKeyDown = (event: KeyboardEvent) => {
     if (
       event.key === "Backspace" &&
-      (event.target as HTMLElement)?.nodeName !== "INPUT"
+      (event.target as HTMLElement).nodeName !== "INPUT"
     ) {
       for (var i = elements.length - 1; i >= 0; --i) {
         if (elements[i].isSelected) {