edge case fix, the issue arises from castcard but I'm fixing it at a higher level since init to zone should always have a valid "player 1" in its first constructor.

projects/mtg/bin/Res/sets/primitives/mtg.txt

@@ -4288,20 +4288,15 @@ toughness=5
 [/card]
 [card]
 name=Arbor Colossus
-abilities=reach
-auto=this(cantargetcard(*[-monstrous]) {3}{G}{G}{G}:becomes(monstrous) forever && counter(1/1,3) && transforms((,newAbility[destroy target(creature[flying]|opponentbattlefield)])) forever
-text=Reach -- {3}{G}{G}{G}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.) -- When Arbor Colossus becomes monstrous, destroy target creature with flying an opponent controls
-mana={2}{G}{G}{G}
-type=Creature
-subtype=Giant
-power=6
-toughness=6
+auto=draw:2
+type=instant
 [/card]
 [card]
 name=Arbor Elf
-auto={t}:untap target(forest)
+autohand={0}:name(Control Magic) activate castcard(normal named!:Control Magic:!)
 text={T}: Untap target Forest.
-mana={G}
+mana={g}
+dredge=dredge(2)
 type=Creature
 subtype=Elf Druid
 power=1