When I remove the last input belt (even after removing the belt balancer itself, and even if I then replace it with another belt), I get the following crash:

The mod Belt Balancer (3.0.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer::on_nth_tick(8)
LuaTransportLine API call when LuaTransportLine was invalid.
stack traceback:
    [C]: in function '__len'
    __belt-balancer__/objects/balancer.lua:198: in function 'run'
    __belt-balancer__/helper/message-handler.lua:33: in function <__belt-balancer__/helper/message-handler.lua:31>

example set up:
("=" = belt; "@" = balancer; "->" = direction of flow)

-> ==@== ->
    ^ remove this belt and you get a crash