Cannot import/clone courses with quizzes when the Yoast SEO plugin is active

[eri-trabiccolo]

Reproduction Steps

• Make sure the Yoast SEO plugin is active
• Import a course with a quiz, e.g. the LifterLMS Quickstart Course

Expected Behavior

• Import should go through

Actual Behavior

• Import hangs and error is logged

This is something expected actually, as it's similar to the #1530 issue.
I thought about it tonight and verified it now.

Error Messages / Logs

[19-Feb-2021 08:49:22 UTC] PHP Fatal error:  Uncaught Error: Call to a member function get() on bool in /shared/httpd/mywp/wordpress.git/wp-content/plugins/lifterlms/templates/quiz/start-button.php:26
Stack trace:
#0 /shared/httpd/mywp/wordpress.git/wp-content/plugins/lifterlms/includes/functions/llms.functions.template.php(120): include()
#1 /shared/httpd/mywp/wordpress.git/wp-content/plugins/lifterlms/includes/functions/llms.functions.templates.quizzes.php(217): llms_get_template('quiz/start-butt...')
#2 /shared/httpd/mywp/wordpress.git/wp-includes/class-wp-hook.php(292): lifterlms_template_start_button('')
#3 /shared/httpd/mywp/wordpress.git/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters(NULL, Array)
#4 /shared/httpd/mywp/wordpress.git/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#5 /shared/httpd/mywp/wordpress.git/wp-content/plugins/lifterlms/templates/content-single-quiz-after.php(11): do_action('lifterlms_singl...')
#6 /shared/httpd/mywp/wordpress.git/wp-includes/template.php(732): require('/shared/httpd/m...')
#7 /shared/htt in /shared/httpd/mywp/wordpress.git/wp-content/plugins/lifterlms/templates/quiz/start-button.php on line 26

I think we need to fix this in the template as well, because I guess we can have this issue when retrieving an orphan quiz's content (rendered sub-prop) via rest-api too.

System and Environment Information

System Report

LifterLMS 4.16.0
Yoast SEO 15.8

This issue has been recreated:

• Locally
• On a staging site
• On a production website
• With only LifterLMS and a default theme

Browser, Device, and Operating System Information

n/a