Behat extension issue when printing current url

I recently began learnign Behat and performed some basic tests successfully. Now I was to perform test more suffisticated tests such as determining the current page url but I get the following error when I execute behat:

When I visit the drupal user registration page "https://ift.tt/3bvN94W" # FeatureContext::iVisitTheDrupalUserRegistrationPage() Mink instance has not been set on Mink context class. Have you enabled the Mink Extension? (RuntimeException)

Feature: Testing Test site
  Testing pages etc on the Test site
    
  Scenario:
    Given I an an anonymous user
    When I visit the drupal user registration page "https://test.server.co.uk/store/user/register"
    And I press "Create new account"
    Then I should see "Username field is required" in page output.
    Then I should see "E-mail address field is required" in page output.
    Then I should see "Password field is required" in page output.
    Then I should see "Surname field is required" in page output.
    Then I should see "Address Line 1 field is required" in page output.
    Then I should see "County / State field is required" in page output.
    Then I should see "First name field is required" in page output.
    Then I should see "Postcode field is required" in page output.
    Then I should see "Telephone field is required" in page output.
    Then I should see "Town field is required" in page output.

For which I have the following php code :

<?php

use Behat\Behat\Context\Context;
use Behat\Behat\Tester\Exception\PendingException;
use Behat\Gherkin\Node\PyStringNode;
use Behat\Gherkin\Node\TableNode;
use Behat\MinkExtension\Context\RawMinkContext;

/**
 * Defines application features from the specific context.
 */
class FeatureContext extends RawMinkContext implements Context
{
    private $client;
    private $response;

    public function checkContent($needle='', $haystack='') {
        return strpos($haystack, $needle);
    }

    /**
     * Initializes context.
     *
     * Every scenario gets its own context instance.
     * You can also pass arbitrary arguments to the
     * context constructor through behat.yml.
     */
    public function __construct()
    {
        $this->client = new \GuzzleHttp\Client(['base_uri' => 'https://test.server.co.uk/']);
    }

    /**
     * @Given I an an anonymous user
     */
    public function iAnAnAnonymousUser()
    {
        $this->response = $this->client->get('/');
        $responseCode = $this->response->getStatusCode();
        if( $responseCode != 200 )
            throw new Exception('Cannot reach site');

        return true;
    }


    /**
     * @When I visit the drupal user registration page :registrationUrl
     */
    public function iVisitTheDrupalUserRegistrationPage($registrationUrl)
    {
        print "--------------------TESTING---------------------- : " . $this->getSession()->getCurrentUrl();
    }

    /**
     * @When I press :arg1
     */
    public function iPress($arg1)
    {
        return true;
//        $this->getSession()->getPage()->find('css', '#edit-submit')->submit();
    }

    /**
     * @Then I should see :errors in page output.
     */
    public function iShouldSeeInPageOutput($errors)
    {
    }
}

In the iVisitTheDrupalUserRegistrationPage function I am trying to get the current url which results in the error I indicated above.

I am not sure what the source of the error is. This post suggests the issue is to do with my behat.yml setup, included below, but the issue still persists after I made corrections :

default:
  suites:
    default:
      contexts:
        - FeatureContext
        - AnotherContext
  extensions:
    Behat\MinkExtension:
      base_url: https://test.server.co.uk
      sessions:
        default:
          goutte: ~

And, I am using the following composer.json configuration :

{
  "require-dev": {
    "behat/behat": "~3.8",
    "behat/mink-extension": "~2.3",
    "behat/mink-goutte-driver": "~1.2",
    "behat/mink-selenium2-driver": "~1.4",
    "guzzlehttp/guzzle" : "~6.0",
    "drupal/drupal-extension" : "~4.1"
  },
  "config": {
    "bin-dir": "bin/"
  }
}

Can anyone help me understand what I'm missing or doing wrong, please ?