Error trying to select a form from symfony2 crawler?

Question

I keep getting the following error:

There was 1 error:

1) Caremonk\MainSiteBundle\Tests\Controller\SecurityControllerFunctionalTest::testIndex
InvalidArgumentException: The current node list is empty.

But when I navigate to the localhost/login, my form populates with the correct content. The line that says...

$form = $crawler->selectButton('login')->form();

is causing the error. What is wrong with my test?

Functional Test:

<?php

namespace Caremonk\MainSiteBundle\Tests\Controller;

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class SecurityControllerFunctionalTest extends WebTestCase
{
    public function testIndex()
    {
        $client = static::createClient();

        $crawler = $client->request('GET', '/login');

        $form = $crawler->selectButton('login')->form();
        $form['username'] = 'testActive';
        $form['password'] = 'passwordActive';
    }
}

Twig view:

{# src/Acme/SecurityBundle/Resources/views/Security/login.html.twig #}
{% if error %}
    <div>{{ error.message }}</div>
{% endif %}

<form action="{{ path('caremonk_mainsite_login_check') }}" method="post">
    <label for="username">Username:</label>
    <input type="text" id="username" name="_username" value="{{ last_username }}" />

    <label for="password">Password:</label>
    <input type="password" id="password" name="_password" />

    {#
        If you want to control the URL the user is redirected to on success (more details below)
        <input type="hidden" name="_target_path" value="/account" />
    #}

    <button type="submit">login</button>
</form>

Answer 1

Answer of Vineet is right but... in my opinion is not good idea to take form from button. Better try:

$crawler->filter('form[name=yourAwesomeFormName]')->form();

or change name for class,id etc. On one view can be more then 1 form with identic submit value

Answer 2

Started doing functional testing in symfony2 and found your question unanswered, so here we go,

selectButton method takes the actual button text as a string argument. So if your button is 'submit my form please', that will be your text.

$form = $crawler->selectButton('submit my form please')->form();

Checkout the answer by Andrew Stackoverflow