I ran into this problem as I was trying to implement another "hide-effect".
Steps to reproduce:
  1. Launch gnome-shell (as you usually do, such as by logging in).
  2. Make sure the dock extension is loaded.
  3. Launch dconf-editor.
  4. Change the value of the /org/gnome/shell/extensions/dock/hide-effect key.

Expected outcome: the hide-effect has been changed.
Actual outcome: the hide-effect has been left unchanged.

I've located the source of the problem to the fact that the "leave_event" and "enter_event" variables are uninitialized when first running the signal handler for the "changed" event for the dconf key mentioned above.

Please find attached a patch to solve the problem.

PS: you may see that upon changing the dconf key, your dock gets moved. This is because of a line I'm not sure is necessary, which does 'this.actor.y = 0'. However, the hide-effect is left unchanged.