Error: Notice: Undefined index: host

I encounter this error, please have a look.
#0 <b>{main}</b> at C:\inetpub\wwwroot\EducCave\wp-content\plugins\easy-registration-forms\includes\class-do-actions.php:208
#1 <b>ERForms_Do_Actions::allowed_redirect_hosts()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:290
#2 <b>WP_Hook::apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\plugin.php:206
#3 <b>apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\pluggable.php:1448
#4 <b>wp_validate_redirect()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\functions.php:1848
#5 <b>wp_get_referer()</b> at C:\inetpub\wwwroot\EducCave\wp-content\plugins\bbpress\includes\common\functions.php:1531
#6 <b>bbp_logout_url()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:288
#7 <b>WP_Hook::apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\plugin.php:206
#8 <b>apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\general-template.php:371
#9 <b>wp_logout_url()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\admin-bar.php:314
#10 <b>wp_admin_bar_my_account_menu()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:288
#11 <b>WP_Hook::apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:312
#12 <b>WP_Hook::do_action()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\plugin.php:544
#13 <b>do_action_ref_array()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\admin-bar.php:86
#14 <b>wp_admin_bar_render()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:288
#15 <b>WP_Hook::apply_filters()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\class-wp-hook.php:312
#16 <b>WP_Hook::do_action()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\plugin.php:478
#17 <b>do_action()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\general-template.php:2895
#18 <b>wp_footer()</b> at C:\inetpub\wwwroot\EducCave\wp-content\themes\edumodo-child\footer.php:292
#19 <b>require_once()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\template.php:722
#20 <b>load_template()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\template.php:671
#21 <b>locate_template()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\general-template.php:76
#22 <b>get_footer()</b> at C:\inetpub\wwwroot\EducCave\wp-content\themes\edumodo-child\front-page.php:135
#23 <b>include()</b> at C:\inetpub\wwwroot\EducCave\wp-includes\template-loader.php:98
#24 <b>require_once()</b> at C:\inetpub\wwwroot\EducCave\wp-blog-header.php:19