BPS Pro 12.1 has been released with the corrected code. Please upgrade to BPS Pro 12.1. Thanks for reporting this bug. 😉
BugFix: Plus sign + operand code changed to array_merge() in the Pre-Installation Wizard AutoRestore Exclude wp-content Folders code:
$uniqueDirs = array_merge($uniqueDirs1, $plugins_dir, $mu_plugins_dir);