I'm not quire sure how to reproduce this. You just need to make sure the "selectactivities" index in the config data is not set in the block configuration in the course before performing a restore.

Error messages during restore:

PHP Notice: Undefined index: selectactivities in /moodle/blocks/completion_progress/backup/moodle2/restore_completion_ progress_block_task.class.php on line 45

Notice: Undefined index: selectactivities in /moodle/blocks/completion_progress/backup/moodle2/restore_completion_progr ess_block_task.class.php on line 45
PHP Warning: Invalid argument supplied for foreach() in /moodle/blocks/completion_progress/backup/moodle2/restore_comp letion_progress_block_task.class.php on line 45

Warning: Invalid argument supplied for foreach() in /moodle/blocks/completion_progress/backup/moodle2/restore_completio n_progress_block_task.class.php on line 45

This pull request solves the problem:
https://github.com/deraadt/moodle-block_completion_progress/pull/37