Error message

• Notice: Trying to access array offset on value of type null in forum_get_topics() (line 917 of /home2/mat1990/public_html/modules/forum/forum.module).
• Deprecated function: implode(): Passing glue string after array is deprecated. Swap the parameters in drupal_get_feeds() (line 385 of /home2/mat1990/public_html/includes/common.inc).