When there is a missing label (here OffBad at last line), PlantUml crash:
@startuml BugTiming
robust "Main Switch" as MainSwitch
MainSwitch is Off
@0 as :On
@5 as :Off
@:On
MainSwitch is On
@:Off
MainSwitch is Off
MainSwitch@:On <-> @:OffBad : tOn
@enduml
See on PlantUML Server