It seems that the crash is related to all of the "Write" events. I tested the Event ChaosDestruction Write, Event Float Write, and Event Bool Write and all 3 caused the crash, but their "Read" counterparts did not cause a crash.
Creating an empty Niagara System and adding these doesn't cause a crash, so this only occurs when there is an emitter in the System.
Tested in //UE4/Release-4.25 @ CL 14469661 and this occurred, so not marking this bug as a Regression.
The engine crashes
The user would be able to add the Event Bool Write node without crashing
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-115658 in the post.