PHP8.1 strtolower error with null value

[krissss]

Prerequisites

• [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [x] The issue still exists against the latest master branch of yii2-dynagrid.
• [x] This is not an usage question. I confirm having gone through and read the documentation and demos.
• [x] This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• [x] I have attempted to find the simplest possible steps to reproduce the issue.
• [x] I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

php8.1

Expected behavior and actual behavior

When I follow those steps, I see...

Environment

PHP

• [x] 8.1

Browsers

• [x] Google Chrome
• [x] Mozilla Firefox
• [x] Internet Explorer
• [x] Safari

Operating System

• [x] Windows
• [x] Mac OS X
• [x] Linux
• [x] Mobile

Libraries

• jQuery version:
• yii2-dynagrid version: v1.5.3

Isolating the problem

• [x] This bug happens on the demos page
• [x] The bug happens consistently across all tested browsers
• [x] This bug happens when using yii2-dynagrid without other plugins.