Factorio (version 1.1.12 build 57713, linux64) crashed after I pressed control-c and clicked and dragged around some entities, intending to copy them.

In the logs I see:

Error MainLoop.cpp:1285: Exception at tick 4208563: The mod Pipelayer (0.5.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event pipelayer::on_player_setup_blueprint (ID 68)
__pipelayer__/lualib/BaseEditor/BaseEditor.lua:435: attempt to index local 'entity' (a nil value)
stack traceback:
  __pipelayer__/lualib/BaseEditor/BaseEditor.lua:435: in function 'proxy_name'
  __pipelayer__/lualib/BaseEditor/BaseEditor.lua:699: in function 'convert_bp_entities_to_bpproxies'
  __pipelayer__/lualib/BaseEditor/BaseEditor.lua:771: in function 'capture_underground_entities_in_blueprint'
  __pipelayer__/Editor.lua:593: in function 'on_player_setup_aboveground_blueprint'
  __pipelayer__/Editor.lua:611: in function 'on_player_setup_blueprint'
  __pipelayer__/control.lua:43: in function <__pipelayer__/control.lua:43>

Along with other reports of crashes caused by this mod's interaction with blueprints, it seems that the problems seem to stem from trying to use nil values. Is there any way that this mod could check to make sure these values are not nil before trying to access them?

Could you post a save?

There are several hundred places where a nil check could be missing. But it's usually a bad idea to try to throw nil checks everywhere, because it usually indicates a different problem elsewhere in the code.