This was triggered with a basic operation of placing a Realistic Fusion Power (RFP) heat exchanger. Initially it was right next to a fusion reactor but on a second launch of the game, I placed the heat exchanger by itself to trigger the same error.

The mod Reactor Interface (2.1.1) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event Reactor Interface::on_tick (ID 0)
Reactor Interface/control.lua:337: attempt to index local 'burner' (a nil value)
stack traceback:
Reactor Interface/control.lua:337: in function <Reactor Interface/control.lua:322>