Error while running event mining-patch-planner::on_player_cursor_stack_changed (ID 31)
mining-patch-planner/gui.lua:264: attempt to index field 'flags' (a nil value)
stack traceback:
mining-patch-planner/gui.lua:264: in function 'update_pole_selection'
mining-patch-planner/gui.lua:322: in function 'update_selections'
mining-patch-planner/gui.lua:336: in function 'show_interface'
mining-patch-planner/control.lua:73: in function <mining-patch-planner/control.lua:69>

I just try to use it. Downgrade to 1.2.5, all works fine

Thanks for the report, my bad for the regression. By any chance do you have a savefile that crashes when updating to 1.2.6?
I think I know what went wrong but I would like to confirm.

Released a hotfix version 1.2.7, please report if it still crashes.