When targetids in ctemplate->unlink is set to null, the generated SQL statement fails because the following statement is generated:

SELECT DISTINCT g.graphid,g.name,g.flags,h.name as host FROM graphs g WHERE EXISTS (SELECT ggi.graphid FROM graphs_items ggi,items ii WHERE ggi.graphid=g.templateid AND ii.itemid=ggi.itemid AND (ii.hostid IN ('10002')) ) AND (g.flags IN ('0','1'))

Because h is not included in all cases (=when targetids is null), the SQL statement fails. As I honestly have no idea what happens where and why due to no documentation, I have no idea how to fix that. For my bug reporting issues, I simply have set the following near line 2092:

$sql_from = ' graphs g,hosts h';