Sometimes following error has showing?

[Kinaiken]

W/System.err: java.lang.IllegalStateException: Fragment FolioPageFragment{f42573a} not attached to Activity
W/System.err: at android.support.v4.app.Fragment.getResources(Fragment.java:646)
W/System.err: at android.support.v4.app.Fragment.getString(Fragment.java:668)
W/System.err: at com.folioreader.fragments.FolioPageFragment$4.onJsAlert(FolioPageFragment.java:311)
W/System.err: at com.android.webview.chromium.WebViewContentsClientAdapter.handleJsAlert(WebViewContentsClientAdapter.java:820)
W/System.err: at org.chromium.android_webview.AwContentsClientBridge.handleJsAlert(AwContentsClientBridge.java:233)
W/System.err: at org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
W/System.err: at org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:41)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err: at android.os.Looper.loop(Looper.java:148)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:7325)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)