PHP 8.1 has deprecating passing NULLs to many string manipulating functions. This can cause contrib tests to fail with:
htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated.
This is due to FormattableMarkup replacements being null - those go straight as nulls down to Html::escape() and hence to htmlspecialchars(), where PHP 8.1 complains.
The PHP deprecation can be really hard to debug because you don't get a stack trace. Therefore, this issue adds a deprecation to FormattableMarkup that tells you which string and argument key is causing the problem.
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Html::escape and Html::decodeEntities are typehinted to return string. This is okay because these are static functions.
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Passing a NULL value as a placeholder value to translatable markup or formattable markup is deprecated and will cause an exception in Drupal 11.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
A custom entity type without a label will trigger the error mentioned by @ghost-of-drupal-past when using a DraggableListBuilder and when saving the entity using EntityForm. I'll open a separate issue, but the solution might be a rollback of this change.