JINテーマのウィジェットのアイコン画像が変更できない【解決できた】

Fatal error: Uncaught Error: Call to undefined function is_plugin_active() in /home/mtgolden1/d-life-mt.com/public_html/wp-content/plugins/rich-table-of-content/functions.php:519 Stack trace: #0 /home/mtgolden1/d-life-mt.com/public_html/wp-content/plugins/rich-table-of-content/functions.php(653): rtoc_get_index('<p>\xE3\x82\x81\xE3\x81\xA3\xE3\x81\xA1\xE3\x82\x83...') #1 /home/mtgolden1/d-life-mt.com/public_html/wp-includes/class-wp-hook.php(307): rtoc_switch_mokuji('<p>\xE3\x82\x81\xE3\x81\xA3\xE3\x81\xA1\xE3\x82\x83...') #2 /home/mtgolden1/d-life-mt.com/public_html/wp-includes/plugin.php(191): WP_Hook->apply_filters('<p>\xE3\x82\x81\xE3\x81\xA3\xE3\x81\xA1\xE3\x82\x83...', Array) #3 /home/mtgolden1/d-life-mt.com/public_html/wp-includes/post-template.php(253): apply_filters('the_content', '\xE3\x82\x81\xE3\x81\xA3\xE3\x81\xA1\xE3\x82\x83\xE6\x8B\x98...') #4 /home/mtgolden1/d-life-mt.com/public_html/wp-content/themes/jin/single.php(81): the_content() #5 /home/mtgolden1/d-life-mt.com/public_html/wp-includes/template-loader in /home/mtgolden1/d-life-mt.com/public_html/wp-content/plugins/rich-table-of-content/functions.php on line 519