Issue title: TestPayment with status "Open": Call to a member function isOpen() on null ..
 Issue body: Hello,

The plugin is crashing (error 500) while doing a TestPayment with the status "Open", all other statuses are fine, it worked in v4.0.2, but got broken in v5.0.0 ..

```
8542#0: *549597 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Call to a member function isOpen() on null in [..]/httpdocs/wp-content/plugins/mollie-payments-for-woocommerce/includes/mollie/wc/gateway/abstract.php:1415
Stack trace:
#0 [..]/httpdocs/wp-content/plugins/mollie-payments-for-woocommerce/includes/mollie/wc/plugin.php(262): Mollie_WC_Gateway_Abstract->getReturnRedirectUrlForOrder(Object(WC_Order))
#1 [..]/httpdocs/wp-includes/class-wp-hook.php(286): Mollie_WC_Plugin38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5onMollieReturn('')
#2 [..]/httpdocs/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)
#3 [..]/httpdocs/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#4 [..]/httpdocs/wp-content/plugins/woocommerce/includes/class-wc-api.php(97): do_action('woocommerce_api...')
#5 [..]/httpdocs/wp-includes/class-wp-hook.php(286): WC_API->handle_api_requests(Object(WP))
#6 /var/www" while reading response header from upstream		
```


 Comments: 
 Comment 0: Already fixed in the version I'm working on now:
http://take.ms/1b4zl
{'source': 'GithubIssues', 'repo_name_with_owner': 'mollie/WooCommerce', 'issue_id': 369165147, 'repo_stars': 108.0}