How to resolve the following error? "Cannot read properties of undefined (reading 'toLowerCase') – Javascript

I am making a project on React and have come across an error which I have given below.

I am trying to resolve the error but am unable to do so.

Can someone please help?
The error is as follows:
Cannot read properties of undefined (reading ‘toLowerCase’)
TypeError: Cannot read properties of undefined (reading ‘toLowerCase’)

App.js

import "./App.css";
import Navbar from "./Components/Navbar";
import TextForm from "./TextForm";
import React, { useState } from "react";
import Alert from "./Components/Alert";

function App() {
  const [mode, setMode] = useState("light");
  const [alert, setAlert] = useState(null);

  const showAlert = (message, type) => {
    setAlert({
      msg: message,
      type: type,
    });
    setTimeout(() => {
      showAlert(null);
    }, 1000);
  };

  const toggleMode = () => {
    if (mode === "light") {
      setMode("dark");
      document.body.style.backgroundColor = "black";
      showAlert("The dark mode has been set", "success");
    } else {
      setMode("light");
      document.body.style.backgroundColor = "white";
      showAlert("The light mode has been set", "success");
    }
  };
  return (
    <>
      <Navbar toggleMode={toggleMode} mode={mode} />
      <Alert alert={alert} />
      <TextForm
        heading="This is textarea"
        mode={mode}
        alert={alert}
        showAlert={showAlert}
      />
    </>
  );
}

export default App;

Alert.js

import React from "react";

export default function Alert(props) {
  const capitalize = (word) => {
    const lower = word.toLowerCase();
    return lower.charAt(0).toUpperCase() + lower.slice(1);
  };
  return (
    props.alert && (
      <div
        class={`alert alert-${props.alert.type} alert-dismissible fade show`}
        role="alert"
      >
        <strong>{capitalize(props.alert.type)}</strong>:{props.alert.msg}
        <button
          type="button"
          class="btn-close"
          data-bs-dismiss="alert"
          aria-label="Close"
        ></button>
      </div>
    )
  );
}