3.9 version ( 2020041239 ) cannot write files

[goose2000]

Describe the bug
I would really like to continue using Simple Certificate with Moodle 4.0 so I tried to install and knew it would probably have problems but I want to report - hoping I might get some insight on fixing it for M4.x

In essence, it all installs and is configurable, but it has trouble writing files or temp file I believe:
Error code: storedfilenotcreated

Moodle Version(s)
Moodle 4.0 (Build: 20220419) - PHP 7.4 - MySQL 8.0

To Reproduce
Steps to reproduce the behavior:

1. Install version for moodle 3.9 from github branch - on a fresh Moodle 4.0 instance
2. Create example Simple Certificate activity
3. Try to view from "Issue a test certificate" tab - click Get Certificate
4. Here is the debug info from Moodle :

Coding error detected, it must be fixed by a programmer: PHP catchable fatal error

More information about this error

×Debug info: Object of class stored_file could not be converted to string
Error code: codingerror
×Stack trace:
line 427 of \lib\setuplib.php: coding_exception thrown
line 2315 of \lib\filelib.php: call to default_error_handler()
line 2602 of \lib\filelib.php: call to readfile_accel()
line 2756 of \lib\filelib.php: call to send_file()
line 1325 of \mod\simplecertificate\locallib.php: call to send_stored_file()
line 1911 of \mod\simplecertificate\locallib.php: call to simplecertificate->output_pdf()
line 122 of \mod\simplecertificate\view.php: call to simplecertificate->view_default()

Expected behavior
Open a PDF certificate in a pop-up window.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• Windows
• Chrome

Additional context
Add any other context about the problem here.

[danko-nobre]

Tracing back the error stack, it seems that moodle's debbugger is throwing an error 'cause is not possible to generate a file and show it in the same call (buffer error).

Disabling the debug messages in "/admin/settings.php?section=debugging" seems to avoid this problem.

[leonstr]

This issue is complicated by the code to the handle errors in readfile_accel() containing an error, see MDL-72582. The Object of class stored_file could not be converted to string message is this second error within Moodle, not the error causing the problem.