avoid deprecation errors along the lines of:

[PHP Deprecation] explode(): Passing null to parameter #2 ($string) of type string is deprecated at /var/www/powerbase/sites/all/modules/civicrm/CRM/Utils/Mail/EmailProcessor.php:83

CRM/Utils/Mail/EmailProcessor.php

@@ -80,9 +80,9 @@ public static function processActivities() {
   private static function _process($civiMail, $dao, $is_create_activities) {
     // 0 = activities; 1 = bounce;
     $isBounceProcessing = $dao->is_default;
-    $targetFields = array_filter(explode(',', $dao->activity_targets));
-    $assigneeFields = array_filter(explode(",", $dao->activity_assignees));
-    $sourceFields = array_filter(explode(",", $dao->activity_source));
+    $targetFields = array_filter(explode(',', (string) $dao->activity_targets));
+    $assigneeFields = array_filter(explode(",", (string) $dao->activity_assignees));
+    $sourceFields = array_filter(explode(",", (string) $dao->activity_source));
     // create an array of all of to, from, cc, bcc that are in use for this Mail Account, so we don't create contacts for emails we aren't adding to the activity.
     $emailFields = array_merge($targetFields, $assigneeFields, $sourceFields);
     $createContact = !($dao->is_contact_creation_disabled_if_no_match) && !$isBounceProcessing;