I would suggest that if a variable is to be used, it should be defined, no matter what the error reporting level.
This warning manifests itself when an operator is added with an image to be stored as a blob.
In hcl/class/operator.php,
replace code on line 196
if ($blob === '') $blob = ' ';
with
if( isset($blob) ) {
if ($blob === '') $blob = ' ';
}