Fix missing class import and null check to ensure module functionality

I creaed the MR.

Please review.

I have reviewed the fix and tested the module functionality in a clean Drupal installation. In that fresh Drupal site, I installed the module with the patched changes from this issue and then enabled it.

Steps to test

1. Installed and enabled the module successfully with no errors
2. Created a new poll with multiple choices
3. Voted as administrator
4. Verified that the vote was successfully recorded and displayed in the results without any errors

I am attaching screenshots that demonstrate its correct functionality.