This happens after I remove this mod from save https://mods.factorio.com/mod/CircuitHUD-V2
I had to remove it because I had this bug with it https://mods.factorio.com/mod/UtilizationMonitorBlargh/discussion/601a07ccf5bdb41ee3c4791b (reported to author).

However, now UM crashed on it's own on game save load.

The mod Utilization Monitor (Blargh) (0.8.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UtilizationMonitorBlargh::on_init()
UtilizationMonitorBlargh/control.lua:243: attempt to index field 'filter' (a nil value)
stack traceback:
UtilizationMonitorBlargh/control.lua:243: in function 'add_entity'
UtilizationMonitorBlargh/control.lua:366: in function 'reset'
UtilizationMonitorBlargh/control.lua:425: in function <UtilizationMonitorBlargh/control.lua:424>

Interesting - same location as the other person - this happens in the code where UM is trying to calculate the maximum performance capability for a boiler type object. I'm getting ready to take a look at the mod he pointed to as a possibility - what other mods do you have that might be doing something similar (specifically, adding boilers that don't have a fluid connection may be the cause)

Actually, at line 243, I'm guessing this would be a boiler that DOES a fluid connection, but can take any type of fluid in? (There's no filter applied to the boiler's fluid intake).

I'm adding a check for it in 0.8.6, but I'd like to figure out what mod it is that has such a beast so I can see if its something that needs to be monitored, and if so, how. If you can let me know what mods you're running (or just a new game is probably easier), that'd be helpful.

0.8.6 is posted which SHOULD fix the problem, but I was unable to reproduce it. Let me know if it's fixed for you.
Thanks!

Looks like fixed. Thanks man.