動かない

<Router>
        <ul>
          <li>
            <Link to="/">Home</Link>
          </li>
          <li>
            <Link to="/about">About</Link>
          </li>
          <li>
            <Link to="/dashboard">Dashboard</Link>
          </li>
        </ul>

        <hr />

        {/*
          A <Switch> looks through all its children <Route>
          elements and renders the first one whose path
          matches the current URL. Use a <Switch> any time
          you have multiple routes, but you want only one
          of them to render at a time
        */}
        <Switch>
          <Route exact path="/">
            <Home />
          </Route>
          <Route path="/about">
            <About />
          </Route>
          <Route path="/dashboard">
            <Dashboard />
          </Route>
        </Switch>
    </Router>

動く

<Router>
      <div>
        <ul>
          <li>
            <Link to="/">Home</Link>
          </li>
          <li>
            <Link to="/about">About</Link>
          </li>
          <li>
            <Link to="/dashboard">Dashboard</Link>
          </li>
        </ul>

        <hr />

        {/*
          A <Switch> looks through all its children <Route>
          elements and renders the first one whose path
          matches the current URL. Use a <Switch> any time
          you have multiple routes, but you want only one
          of them to render at a time
        */}
        <Switch>
          <Route exact path="/">
            <Home />
          </Route>
          <Route path="/about">
            <About />
          </Route>
          <Route path="/dashboard">
            <Dashboard />
          </Route>
        </Switch>
      </div>
    </Router>

routerの中が単一の要素でないと動かないらしい。