What's this error, why happens?

05-17 18:24:57.069: ERROR/WindowManager(18850): Activity com.mypkg.myP has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@44c46ff0 that was originally added here
05-17 18:24:57.069: ERROR/WindowManager(18850): android.view.WindowLeaked: Activity ccom.mypkg.myP has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@44c46ff0 that was originally added here
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.view.ViewRoot.<init>(ViewRoot.java:231)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:148)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.view.Window$LocalWindowManager.addView(Window.java:424)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.Dialog.show(Dialog.java:239)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at com.mypkg.myP$PreparePairingLinkageData.onPreExecute(viewP.java:183)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.os.AsyncTask.execute(AsyncTask.java:391)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at com.mypkg.myP.onCreate(viewP.java:94)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2544)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2621)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.ActivityThread.access$2200(ActivityThread.java:126)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1932)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.os.Looper.loop(Looper.java:123)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at android.app.ActivityThread.main(ActivityThread.java:4595)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at java.lang.reflect.Method.invokeNative(Native Method)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at java.lang.reflect.Method.invoke(Method.java:521)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
05-17 18:24:57.069: ERROR/WindowManager(18850):     at dalvik.system.NativeStart.main(Native Method)