Took me a while to figure this one out...
In my case it was because a breakpoint that I had set was no longer in a valid line of code.
This is a terrible bug. Just because a breakpoint can't be set, the whole debugger should not fall flat on its face.
Invalid breakpoints should be simply ignored skipped over on failure. Every other (good) debugger in the world *silently* ignores invalid breakpoints.
Devs all know they need to clean up old breakpoints periodically, but don't *force* me to do it.