Incompatibility of metabox in admin pages when moving to PHP 8

[SUGO-es]

Type of issue

• [x] Bug report.
• [ ] Suggestion.

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 dev branch.
• [ ] This is not a usage question (Those should be directed to the community).
• [x] I have attempted to find the simplest possible steps to reproduce the issue.

Steps to reproduce the issue (for Bug report)

1. Look at the code I give you below, run it in PHP 7.4, the code works fine.
2. Switch to PHP 8, in my case I am testing on PHP 8.0.3
3. The metabox will stop rendering ...

Expected behavior and actual behavior

I am looking at possible incompatibilities to move some of my clients from php 7.4 to php 8, the tests I have done so far show me a significant increase in performance and are encouraging me to make the move from PHP 7.4 to PHP 8.0.3.

When changing the version of php I realized that the metaboxes that I had created in pages without tabs disappeared, the pages that have tabs, curiously, render them well.

I have tried searching within the code but can't find the reason.

Could you look at this incompatibility with PHP 8 what is it due to?

I am not getting any php error or warning, I find it very strange ...

I attach a small plugin with an example of yours on how metaboxes work on pages:

test-page-metabox.zip

Environment

Browsers

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

Operating System

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

Versions

• Admin Page Framework version: 3.8.27
• WordPress version: 5.7.2
• PHP version: 8.0.3

Isolating the problem

• [x] This bug happens on the demo page.
• [x] The bug happens consistently across all tested browsers.
• [x] This bug happens with clean install (with the default theme and no plugin except the problematic one).

[michaeluno]

I could confirm this behavior.

Thanks for the report.

[SUGO-es]

Hi micheluno,

I have fixed this solution in the base plugin and in my compilation and everything seems to work correctly.

Thank you very much for your prompt attention.

Best regards..