Prevent canceling of subtasks from creating null description subtasks

http://ideaflowdx.openmastery.org/ideaflow/timeline/task/75/full

<img width="1229" alt="screenshot 2017-04-27 21 10 28" src="https://cloud.githubusercontent.com/assets/2080316/25510522/c3ea6490-2b8e-11e7-95a0-d31d1e827d05.png">

<img width="411" alt="screenshot 2017-04-27 21 16 45" src="https://cloud.githubusercontent.com/assets/2080316/25510537/da2ca79a-2b8e-11e7-98e0-fe80965aad86.png">
