Error when turning to the human-AI battle page

[calcitem]

Error: This widget has been unmounted, so the State no longer has a context (and should be considered defunct). 
Consider canceling any active work during "dispose" or using the "mounted" getter to determine if the State is still active.
 Stack trace: 

#0 State.context. (package:flutter/src/widgets/framework.dart:927:9) 
#1 State.context (package:flutter/src/widgets/framework.dart:933:6) 
#2 _GamePageState.showTips (package:sanmill/widgets/game_page.dart:76:9) 
#3 _GamePageState.engineToGo (package:sanmill/widgets/game_page.dart:239:9)