Don't attempt to configure a dismissed dialog during Android activity

reloading

As part of automatic activity reloading following a configuration
change, Android invokes Activity#onPrepareDialog() even for dismissed
dialogs. Consequently, one can't make the assumption that this method
is only invoked by explicit calls to Activity#showDialog() from our
code.

The actual problem here was the mActiveMessages member being null
at such times.

src/com/fsck/k9/activity/MessageList.java

@@ -1293,6 +1293,14 @@ public class MessageList
         return super.onCreateDialog(id);
     }
 
+    /*
+     * (non-Javadoc)
+     *
+     * Android happens to invoke this method even if the given dialog is not
+     * shown (eg. a dismissed dialog) as part of the automatic activity
+     * reloading following a configuration change (orientation, keyboard,
+     * locale, etc.).
+     */
     @Override
     public void onPrepareDialog(final int id, final Dialog dialog) {
         switch (id) {
@@ -1304,10 +1312,16 @@ public class MessageList
             break;
         }
         case R.id.dialog_confirm_spam: {
+            // mActiveMessages can be null if Android restarts the activity
+            // while this dialog is not actually shown (but was displayed at
+            // least once)
+            if (mActiveMessages != null) {
                 final int selectionSize = mActiveMessages.size();
                 final String message;
-            message = getResources().getQuantityString(R.plurals.dialog_confirm_spam_message, selectionSize, Integer.valueOf(selectionSize));
+                message = getResources().getQuantityString(R.plurals.dialog_confirm_spam_message, selectionSize,
+                        Integer.valueOf(selectionSize));
                 ((AlertDialog) dialog).setMessage(message);
+            }
             break;
         }
         default: {