PHP 7.2 fatal error in bbp_pre_get_posts_normalize_forum_visibility()

[xknown]

While migrating a site to PHP 7.2, we noticed a new PHP fatal error in the bbp_pre_get_posts_normalize_forum_visibility() function.

[22-Jun-2018 01:32:51 UTC] PHP Fatal error:  Uncaught Error: [] operator not supported for strings in /home/site/public_html/wp-content/plugins/bbpress/includes/forums/functions.php:2197
Stack trace:
#0 /home/site/public_html/wp-includes/class-wp-hook.php(286): bbp_pre_get_posts_normalize_forum_visibility(Object(WP_Query))
#1 /home/site/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)
#2 /home/site/public_html/wp-includes/plugin.php(531): WP_Hook->do_action(Array)
#3 /home/site/public_html/wp-includes/class-wp-query.php(1711): do_action_ref_array('pre_get_posts', Array)
#4 /home/site/public_html/wp-includes/class-wp-query.php(3360): WP_Query->get_posts()
#5 /home/site/public_html/wp-includes/query.php(98): WP_Query->query(Array)
#6 /home/site/public_html/wp-content/plugins/bbpress/includes/common/functions.php(1945): query_posts(Array)
#7 /home/site/public_html/wp-includes/class-wp-hook.php(286): bbp_request_feed_trap(Array)
#8 /home/site/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#9 /home/site/public_html/wp-content/plugins/bbpress/includes/core/sub-actions.php(470): apply_filters('bbp_request', Array)
#10 /home/site/public_html/wp-includes/class-wp-hook.php(286): bbp_request(Array)
#11 /home/site/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#12 /home/site/public_html/wp-includes/class-wp.php(376): apply_filters('request', Array)
#13 /home/site/public_html/wp-includes/class-wp.php(735): WP->parse_request('')
#14 /home/site/public_html/wp-includes/functions.php(995): WP->main('')
#15 /home/site/public_html/wp-blog-header.php(16): wp()
#16 /home/site/public_html/index.php(26): require('/home/site/pub...')
#17 {main}
  thrown in /home/site/public_html/wp-content/plugins/bbpress/includes/forums/functions.php on line 2197

This seems to happen because WP_Query::get() seems to return by default an empty string.

[netweb]

Closing as duplicate of #3031