Hi there!
I am not 100% sure, but - if you really want to delete any deck (including a custom study deck), you may long-press on the deck name in the deck list, and choose "delete deck" from the options.
However, if you were on version 2.15.3 or lower we did fix a bug that may have caused this. You might try making sure you are on 2.15.4 first before deleting things - it might just work!
Finally, it may be a good idea to run "check database" from the top menu in the deck list, just in case there is some corruption in the deck causing it to crash. It really should not crash!
Hope this helps -
-Mike