Warning: strlen() expects parameter 1 to be string, array given

Im getting this messages in my error log:

ErrorException: Warning: strlen() expects parameter 1 to be string, array given
#19 /wp-includes/class-wp-block-parser.php(489): WP_Block_Parser::add_freeform
#18 /wp-includes/class-wp-block-parser.php(267): WP_Block_Parser::proceed
#17 /wp-includes/class-wp-block-parser.php(236): WP_Block_Parser::parse
#16 /wp-includes/blocks.php(521): parse_blocks
#15 /wp-includes/blocks.php(274): filter_block_content
#14 /wp-includes/formatting.php(4924): wp_pre_kses_block_attributes
#13 /wp-includes/class-wp-hook.php(288): WP_Hook::apply_filters
#12 /wp-includes/plugin.php(206): apply_filters
#11 /wp-includes/kses.php(914): wp_kses_hook
#10 /wp-includes/kses.php(737): wp_kses
#9 /wp-includes/kses.php(1941): wp_kses_post
#8 /wp-content/plugins/ninja-forms/includes/MergeTags/Other.php(54): NF_MergeTags_Other::init
#7 /wp-includes/class-wp-hook.php(288): WP_Hook::apply_filters
#6 /wp-includes/class-wp-hook.php(312): WP_Hook::do_action
#5 /wp-includes/plugin.php(478): do_action
#4 /wp-settings.php(523): require_once
#3 /wp-config.php(182): require_once
#2 /wp-load.php(37): require_once
#1 /wp-blog-header.php(13): require
#0 /index.php(17): null