We would need the full crash to feed into a decoder and figure out if you're matching the bug you mentioned or not.
Regarding debugging - the issue seems fairly random, at least as described in the bug, there isn't really much more that can be done than get the crashlog.
The issue is already fixed in FortiManager 7.0.2 and higher, so you could consider an upgrade to see if that helps (7.0.3 is already out).
Other than that, I would suggest a ticket with FortiManager technical support to take a closer look at the crashlog and verify if you're matching the bug you mentioned or if something else is going on.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++