Getting error "Trying to get property of non object" in Process Designer

Summary

Trying to retreive assignee of an incident.

Content

Hi Everyone,

I am trying to create an object listener for the process designer and I just want to know if the incident has ben assigned.

I am trying the following code, but getting this error for that line of code.

if ( gettype($obj->AssignedTo->Account->ID) != "NULL" || gettype($obj->AssignedTo->StaffGroup->ID) != "NULL"  ) {

$assigned = true;

}

Fatal error, Trying to get property of non object.

Does anyone know the problem? thanks!!