Symfony 4 - 如果数据库查找失败，如何避免呈现表单？

I have a form which loads based on a lookup from the database of a id value from a get request.

$Id = $request->query->get('id');

if (!empty($Id) && $Id != 'add') {
    $search = $this->getDoctrine()
        ->getRepository(Clients::class)
        ->find($Id);

    if (is_null($search))
        $this->addFlash('danger', 'Invalid Client');
    else
        $form = $this->createForm(ClientViewType::class,$search);
}
else {
    $form = $this->createForm(ClientViewType::class);
}

You can see I'm adding a flashbag message of 'invalid client', but the problem is the form will still show. Is there some way to not show the form? Basically I just want the flashbag message to display and that's it.

I tried some things - i.e. setting $form to null, just returning the page, without the form, etc. but that just forces other problems.

You should indeed set $form to null when you have an invalid client. Then in your twig you could have a conditional rendering like this:

{% if form is not null %}
    {{ form_start(form) }}
        {{ form_widget(form) }}
    {{ form_end(form) }}
{% else %}
    {% for message in app.flashes('danger') %}
        <div class="flash-notice">
            {{ message }}
        </div>
    {% endfor %}
{% endif %}

Write this:

if (is_null($search)) {
    $this->addFlash('danger', 'Invalid Client');
    return $this->render("...", [
       "form" => null
        ...
    ]);
}

and then in twig file make following if condition:

{% if form is not null %}
    {{ form_start(form) }}
         {{ form_widget(form) }}
    {{ form_end(form) }}
{% endif %}

You can use the not famous else clause with the forcondition

{% for message in app.flashes('danger') %}
    <div class="flash-notice">
        {{ message }}
    </div>
{% else %}
   {# your form #}
{% endfor %}

see documentation

Try using instanceof instead.

if ($search instanceof Clients) {...}

This helped me to overcome some of those problems.

I agree with the other answers, especially iiirxs'.