Need to fix the problem "Notice: Undefined offset: 4"

Problem/Motivation

When I tried to remove a signup, appear this error: Notice: Undefined offset: 4 in Drupal\simply_signups\Form\Confirm\SimplySignupsNodesRemoveSingleConfirmForm->buildForm() (line 36 of modules/contrib/simply_signups/src/Form/Confirm/SimplySignupsNodesRemoveSingleConfirmForm.php).

Steps to reproduce

1 - Install Simply Signups;
2 - Select "Basic Page" or "Article" in "Configure" of the Simply Signups;
3 - Create a new "Basic Page" or "Article" Content;
4 - In the created content, in the option "Simply Signups", look for "Settings" option;
5 - Select option "Enabled signups for this node", fill the "Signups open" and "Signups close" fields and save;
6 - Fill the form of Simply Signups in the "View" option;
7 - Open the option "Simply Signups", and try to Remove some item of the list;