Flaxen Intruder's triggered ability goes on the stack without a target. While that ability is resolving, you may sacrifice it. When you do, the reflexive triggered ability triggers and you pick a target artifact or enchantment to be destroyed. This is different from effects that say "If you do . . ." in that players may take actions after you've sacrificed Flaxen Intruder but before the artifact or enchantment is destroyed.