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