Fix an update dialog crash issue.

Stack trace:

```
Exception a1.h: Bad window token, you cannot show a dialog before an Activity is created or after it's hidden.
    at com.afollestad.materialdialogs.MaterialDialog.show (MaterialDialog.java)
    at com.afollestad.materialdialogs.MaterialDialog$Builder.show (MaterialDialog.java:6)
    at org.mewx.wenku8.async.CheckAppNewVersion.onPostExecute (CheckAppNewVersion.java:225)
    at org.mewx.wenku8.async.CheckAppNewVersion.onPostExecute (CheckAppNewVersion.java:15)
    at android.os.AsyncTask.finish (AsyncTask.java:771)
    at android.os.AsyncTask.access$900 (AsyncTask.java:199)
    at android.os.AsyncTask$InternalHandler.handleMessage (AsyncTask.java:788)
    at android.os.Handler.dispatchMessage (Handler.java:106)
    at android.os.Looper.loopOnce (Looper.java:346)
    at android.os.Looper.loop (Looper.java:475)
    at android.app.ActivityThread.main (ActivityThread.java:7889)
    at java.lang.reflect.Method.invoke
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:548)
    at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1009)
```

studio-android/LightNovelLibrary/app/src/main/java/org/mewx/wenku8/async/CheckAppNewVersion.java

@@ -1,5 +1,6 @@
 package org.mewx.wenku8.async;
 
+import android.app.Activity;
 import android.content.Context;
 import android.content.Intent;
 import android.net.Uri;
@@ -77,8 +78,10 @@ protected void onPostExecute(Integer code) {
             if (verboseMode) {
                 Toast.makeText(ctx, ctx.getResources().getString(R.string.system_update_latest_version), Toast.LENGTH_SHORT).show();
             }
-        } else {
-            // update to new version
+        } else if (ctx instanceof Activity && ((Activity) ctx).hasWindowFocus()) {
+            // Note that checking window focus is needed: https://stackoverflow.com/a/41118674/4206925
+
+            // Update to new version.
             new MaterialDialog.Builder(ctx)
                     .theme(Theme.LIGHT)
                     .title(R.string.system_update_found_new)