Go back outside using the flower, Sahiela will be waiting and will tell you to kill her.
Then you go back inside and you just click the heart. (If you hover over it, you'll see it light up and your mouse pointer will change). I just did it like an hour ago, so if this doesn't work, it might be bugged for you guys.
For me, I killed the shadow prince before this (he said nothing about the tree) and also ate his heart with Sebille.
