human
Solve the following python programming problem given the following code:
@ periodic_task.periodic_task 
def _heal_instances(self, ctxt) : 
    "Periodic task to send updates for a number of instances to
        parent cells.

        On every run of the periodic task, we will attempt to sync
        'CONF.cells.instance_update_num_instances' number of instances.
        When we get the list of instances, we shuffle them so that multiple
        nova-cells services aren't attempting to sync the same instances
        in lockstep.

        If CONF.cells.instance_update_at_threshold is set, only attempt
        to sync instances that have been updated recently.  The CONF
        setting defines the maximum number of seconds old the updated_at
        can be.  Ie, a threshold of 3600 means to only update instances
        that have modified in the last hour.
        " 
    if (not self.state_manager.get_parent_cells ()) : 
        return 
info = { 
        'updated_list' : False, 
} 
    def _next_instance() : 
        try : 
            instance = next (self.instances_to_heal) 
except StopIteration : 
            if info ['updated_list'] : 
                return 
threshold = CONF.cells.instance_updated_at_threshold 
            updated_since = None 
            if (threshold > 0) : 
                updated_since = (timeutils.utcnow () - datetime.timedelta (seconds = threshold)) 
self.instances_to_heal = cells_utils.get_instances_to_sync (ctxt, updated_since = updated_since, shuffle = True, uuids_only = True) 
            info ['updated_list'] = True 
            try : 
                instance = next (self.instances_to_heal) 
except StopIteration : 
                return 
return instance 
rd_context = ctxt.elevated (read_deleted = 'yes') 
    for i in range (CONF.cells.instance_update_num_instances) : 
        while True : 
            time.sleep (0) 
            instance_uuid = _next_instance () 
            if (not rd_context) : 
                return 
try : 
                instance = objects.Instance.get_by_uuid (rd_context, instance_uuid) 
except exception.InstanceNotFound :...

    
    

    Fix the buggy line: if (not rd_context) :
assistant
#Here is Python code to fix the buggy line: if (not rd_context) :
.

if (not instance_uuid) :