I have loaded my existing game with this mod, got a warning about trains at stations, set station alert for two trains to -1, clicked the "check" button to apply, and got this:

Error while running event Train_Alerts::on_gui_click (ID 1)
Train_Alerts/script/gui_settings_window.lua:298: attempt to compare number with nil
stack traceback:
Train_Alerts/script/gui_settings_window.lua:298: in function 'reset_timeouts'
Train_Alerts/script/gui_settings_window.lua:285: in function '?'
Train_Alerts/script/gui_settings_window.lua:313: in function 'handler'
Train_Alerts/control.lua:56: in function <Train_Alerts/control.lua:54