I am facing Catchable fatal error

When I open any gallery, it shows following error where tag box should appear:

Catchable fatal error: Object of class WP_Error could not be converted to string in …/wp-content/plugins/nextcellent-gallery-nextgen-legacy/admin/manage/class-ngg-image-list-table.php on line 212

Even if there are several pictures in a gallery but it stops after displaying only the first picture due to this error.

Can anyone pls help me in this regards, thanks