I have been inside the blueprint world (Blueprint Sandboxes mod) and tried copying tiles, when the mod crashed.

The mod Train Control (0.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event train-control::on_player_setup_blueprint (ID 70)
__train-control__/control.lua:255: bad argument #1 of 2 to 'ipairs' (table expected, got nil)
stack traceback:
    [C]: in function 'ipairs'
    __train-control__/control.lua:255: in function <__train-control__/control.lua:243>

Yep, noticed this myself too. I forgot to check if get_blueprint_entities() returned nil. Should be fixed in the latest version.