First off, amazing job on nested subtasks! They're awesome!
Only issue so far is when clicking Create subtask icon, it quite often takes 2 clicks to see the new subtask box. It's not consistent, but if you click the button on a few subtasks, you will see the issue. (Make sure you're clicking on SUBTASKS to create nested subtasks. Also test 2nd and 3rd level subtasks)
This issue doesn't seem to effect the + icon, when there's already subtasks. Only the icon shown in the image.