It might be a bug, but I can't seem to drop tree seeds.
Edit: The way I found to "fix" this is just put in the seed in your inventory, I still can't seem to drop it after, but at least I can continue.
I don't think you're supposed to drop tree seeds. You have to place them either in general storage or a seedling tray to store them. The only way I know of dropping them without storage is to drop the seeds on cereal crop or in grass.
this is fixed in the next version. Thanks