I had this bug now several times, it pops up, when building land around bridges. The reason for that seems to be a another mod (nicefill), cause when I inactivate that the problems go away.

PS: In the end the building of bridges is cool, but quite complex. I would like to buid bridges and then the waterrails.

The mod Cargo Ships caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cargo-ships::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
cargo-ships/logic/bridge_logic.lua:250: in function 'ManageBridges'
cargo-ships/control.lua:355: in function <cargo-ships/control.lua:352>
stack traceback:
[C]: in function 'index'
__cargo-ships/logic/bridge_logic.lua:250: in function 'ManageBridges'
cargo-ships/control.lua:355: in function <cargo-ships/control.lua:352>

thanks for the report, i will look into it.
making bridges possible at all was a pretty large hack, and the requirement too place waterways beforehand was necessarry to deal with engine limitations, and sadly wont go away...