Android - Failure delivering result ResultInfo

 FATAL EXCEPTION: main

ava.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=291, result=801, data=Intent { cmp=com.example.simplecalc/.CalculationTypeActivity (has extras) }} to activity {com.example.simplecalc/com.example.simplecalc.MainActivity}: java.lang.NullPointerException

at android.app.ActivityThread.deliverResults(ActivityThread.java:3367)

at android.app.ActivityThread.handleSendResult(ActivityThread.java:3410)

at android.app.ActivityThread.access$1100(ActivityThread.java:141)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1304)

at android.os.Handler.dispatchMessage(Handler.java:99)

at android.os.Looper.loop(Looper.java:137)

at android.app.ActivityThread.main(ActivityThread.java:5103)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:525)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)

at dalvik.system.NativeStart.main(Native Method)

Caused by: java.lang.NullPointerException

at com.example.simplecalc.MainActivity.onActivityResult(MainActivity.java:49)

at android.app.Activity.dispatchActivityResult(Activity.java:5322)

at android.app.ActivityThread.deliverResults(ActivityThread.java:3363)

... 11 more

錯誤之處通過Log.d()等Log函數定位，發現很簡單的是

calcTypeText.setText(ConstantInfo.typeMap.get(calcType));

中TextView變量calcTypeText沒有和界面TextView對象綁定。

關鍵時注意錯誤信息中的java.lang.NullPointerException，指針為null拋出的異常。