445.850 Error MainLoop.cpp:1204: Exception at tick 662730: The mod Hydrogen Power Systems caused a non-recoverable error.
Please report this error to the mod author.

Error while running event CW-hydrogen-power::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
CW-hydrogen-power/control.lua:265: in function <CW-hydrogen-power/control.lua:255>
stack traceback:
[C]: in function 'index'
__CW-hydrogen-power/control.lua:265: in function <CW-hydrogen-power/control.lua:255>

Let me guess... You're using Klonan's construction drones (or some similar mod) to build a fusion reactor, that's the only way (I think) this error could occur.
Actually I tested if the reactor was valid but I forgot to test its connector was valid but since are both created and destroyed at the same time I didn't have this error until now
Anyway I will include a test in the connector but until Klonan fix his own mod I fear the reactor's connection will be missing if it is built by drones

Actualization: Klonan informed me about an alternative solution I'll try to implement

I built it by hand, and then it got destroyed.

hum... then I guess it got destroyed at the same time it was updating the connector anyway I added the verification please test it