Avoid NPE in MessageOpenPgpView.handleError()

E/AndroidRuntime(25655): FATAL EXCEPTION: main
E/AndroidRuntime(25655): Process: com.fsck.k9, PID: 25655
E/AndroidRuntime(25655): java.lang.NullPointerException
E/AndroidRuntime(25655):     at com.fsck.k9.view.MessageOpenPgpView.handleError(MessageOpenPgpView.java:385)
E/AndroidRuntime(25655):     at com.fsck.k9.view.MessageOpenPgpView.access$3(MessageOpenPgpView.java:384)
E/AndroidRuntime(25655):     at com.fsck.k9.view.MessageOpenPgpView$DecryptVerifyCallback.onReturn(MessageOpenPgpView.java:357)
E/AndroidRuntime(25655):     at org.openintents.openpgp.util.OpenPgpApi$OpenPgpAsyncTask.onPostExecute(OpenPgpApi.java:195)
E/AndroidRuntime(25655):     at org.openintents.openpgp.util.OpenPgpApi$OpenPgpAsyncTask.onPostExecute(OpenPgpApi.java:1)
E/AndroidRuntime(25655):     at android.os.AsyncTask.finish(AsyncTask.java:632)
E/AndroidRuntime(25655):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
E/AndroidRuntime(25655):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
E/AndroidRuntime(25655):     at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(25655):     at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime(25655):     at android.app.ActivityThread.main(ActivityThread.java:5128)
E/AndroidRuntime(25655):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(25655):     at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime(25655):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
E/AndroidRuntime(25655):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611)
E/AndroidRuntime(25655):     at dalvik.system.NativeStart.main(Native Method)
Fix leaked window error in FolderList

Observed after wiping data and then tapping a launcher shortcut
for an account.
Use isFinishing() instead

As suggested by @maniac103
https://github.com/k9mail/k-9/commit/41570e4#commitcomment-7388209