What version of Rockstor are you seeing this on. I know you had both a stable and a testing install at one point?
If you follow the forum link in that issue it gives more info, and a work around, on what was found to occasionally block share (subvol) deletion. Linking to @Haioken and @Rene_Castberg 's thread here for convenience:
I have the lastest version of Rockstor, I removed the non-production edition I had running.
Simply put, I had TWO shares. File and Media. I was able to successfully remove the Files share without issue, when I tried to remove the Media share it gave me the error I shared.
I then tried to remove the POOL and that removed the pool and the share. So not sure what the issue was.