I'm occasionally getting the above error in a save with 2 other players. I'm having trouble reproducing it reliably, it has happened when clicking any button on the flow config UI of a pipe (real, ghost, remote view, underground from AUP).
The only thing I can think of is we might have had multiple players configuring pipes at the same time?
Next time it happens I'll grab a save and see if I can isolate it.

This should be fixed with 2.1.3!

I got a similar error in 2.1.3:

The mod Flow Configuration (2.1.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event flow-config::on_gui_click (ID 1)
LuaPlayer doesn't contain key type.
stack traceback:
    [C]: in function '__index'
    __flow-config__/flowlib/stateutil.lua:50: in function 'is_pipe'
    __flow-config__/control.lua:145: in function 'update_all'
    __flow-config__/control.lua:191: in function 'on_button_direction'
    __flow-config__/control.lua:287: in function <__flow-config__/control.lua:281>

I managed to get a save as this error happened (at least one player had the flow config UI opened, possibly two), but trying to trim down the modlist for debugging just results in the save becoming unloadable, throwing this error:

The mod Flow Configuration (2.1.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event flow-config::on_configuration_changed
LuaPlayer doesn't contain key type.
stack traceback:
    [C]: in function '__index'
    __flow-config__/flowlib/stateutil.lua:50: in function 'is_pipe'
    __flow-config__/control.lua:145: in function 'update_all'
    __flow-config__/control.lua:258: in function <__flow-config__/control.lua:254>