See also
FS#2084: This error occurs with other plugins too, e.g. task plugin.
Beside of this, the function _notify($task) in helper.php seems to be faulty:
line 318:
if ((!$conf['subscribers']) && (!$conf['notify'])) return;
'return' will be reached only, when both subscribers AND notify are 0/undefined. However, 'return' should be executed if subscribers OR notify are 0/undefined. So, the line should read as:
if ((!$conf['subscribers']) || (!$conf['notify'])) return;
line 320:
if ((empty($bcc)) && (!$conf['notify'])) return;
Again, 'return' will be reached only if both conditions are true. Moreover, the repeated test !$conf['notify'] can be omitted, due to (my new) line 318. So, this line should read as:
if (empty($bcc)) return;