how to check that previous route is not null in react router v6?

Better to use

navigate("Your login pathName", { replace: true });

For Example

navigate('/login',{ replace: true })

If I understand your question correctly, you are asking how to redirect a user back to a route they were attempting to access prior to authenticating. It's not really about trying to issue a back navigation, but rather, knowing you came from somewhere and you need to get back to that location.

The solution for this is generally to forward a "from" path, or referrer when redirecting to the login path/component so it can redirect back to the original path.

Here's an example showing the entire UI/UX flow you likely want to achieve. It starts with a specialized route component that grabs the current location and passes that with the redirect to the login route.

import { Navigate, Outlet } from 'react-router-dom';

const PrivateRoutes = () => {
  const location = useLocation(); // <-- gets current location
  const { isAuthenticated } = /* some auth state accessor */;

  return isAuthenticated 
    ? <Outlet />
    : (
      <Navigate
        to="/login"
        replace
        state={{ from: location }} // <-- pass location in state
      />
    );
}

And some protected/unprotected routes.

<BrowserRouter>
  <Routes>
    <Route path="/" element={<PrivateRoutes />} >
      <Route path="/protected" element={<Protected />} />
    </Route>
    <Route path="/login" element={<Login />} />
    <Route path="*" element={<PageNotFound />} />
  </Routes>
</BrowserRouter>

The Login component then handles the authentication flow, and upon successful login, then attempts to redirect back to this target path. Here's where you also provide a fallback path, usually "/", if there is no referrer value.

export default function Login() {
  const location = useLocation();
  const navigate = useNavigate();
  const { login } = /* some auth state accessor */;

  ... local state

  const loginHandler = async () => {
    const isAuthenticated = await login(....);
    if (isAuthenticated) {
      // destructure referrer path or use fallback
      const { from } = location.state || { from: { pathname: "/" } };

      // redirect back
      navigate(from, { replace: true });
    }
  };

  return (
    <div
      style={{ height: "100vh" }}
      className="d-flex justify-content-center align-items-center"
    >
      <button type="button" onClick={loginHandler} className="btn btn-primary">
        Login
      </button>
    </div>
  );
}