In FileMaker Pro Advanced 18.104.22.168 on macOS 10.12.6, issuing a SQL call via a plugin to do something schema-related (e.g., ALTER TABLE or DROP INDEX), consistently sends FileMaker Pro Advanced into a state with the spinning beach ball, and after about 10 seconds, it shows up as "Not responding" in macOS "Force Quit Applications".
Interestingly, after force-quitting and re-opening the crashed file, the SQL call has actually been executed (in other words, a field has been added or renamed, an index dropped, etc).
I've had the exact same results with 5 different plugins:
I haven't tested on Windows, nor on other Macs yet.
The attached file contains sample scripts using Drop Index to try to reproduce the issue. The exact plugin versions used are listed in the script comments (to my knowledge, all are the latest public version at this time).