Problem with "Notice: Undefined index"

Problem/Motivation

When you use the type field "Checkboxes" and have a signup in the list, if you try to change the status of the signup to "checkin" or "checkout", display the error "Notice: Undefined index".

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 "Simply Signups" option, look for "Settings" option;
5 - Select option "Enabled signups for this node", fill the "Signups open" and "Signups close" fields and save;
6 - Open the "Fields" option and "Add field", choose "Checkboxes" in the "Field Type" field, fill the fields required and in the "Allowed values list" field put two or more option values and save;
7 - Fill the form of Simply Signups in the "View" option, save it and go the "Simply Signups" option, in the list of signups have a column "Operations" click on the option "Status" and click in the button "check in" or "check out";
8 - Go to the "Recent log messages" in the "Reports" of Drupal menu.