Amazingly easy. Thanks a lot!
$maintopic is a List here and not a Map so
$maintopic.subtopic1 does
not mean anything.
try with
#set( $subtopic1 = ["one", "two", "three"] )
#set( $subtopic2 = ["one", "two", "three"] )
#set( $maintopic = {"subtopic1" : $subtopic1, "subtopic2" :
$subtopic2} )
Bests,
Moritz