avoid leaking data to MessagingController via MessageLoaderHelper after detach

null handling is already present: no ui methods should ever be called
after onDestroy or onDetachChangingConfigurations, and pending result
callbacks are handled by a null-check for the callback field.

k9mail/src/main/java/com/fsck/k9/activity/MessageLoaderHelper.java

@@ -69,11 +69,11 @@ public class MessageLoaderHelper {
     private static final int DECODE_MESSAGE_LOADER_ID = 2;
 
 
-    // injected state
-    private final Context context;
-    private final FragmentManager fragmentManager;
-    private final LoaderManager loaderManager;
-    @Nullable // may be cleared
+    // injected state - all of this may be cleared to avoid data leakage!
+    private Context context;
+    private FragmentManager fragmentManager;
+    private LoaderManager loaderManager;
+    @Nullable // make this explicitly nullable, make sure to cancel/ignore any operation if this is null
     private MessageLoaderCallbacks callback;
 
 
@@ -126,6 +126,9 @@ public class MessageLoaderHelper {
         }
 
         callback = null;
+        context = null;
+        fragmentManager = null;
+        loaderManager = null;
     }
 
     /** Prevents future callbacks, but retains loading state to pick up from in a call to
@@ -137,6 +140,9 @@ public class MessageLoaderHelper {
         }
 
         callback = null;
+        context = null;
+        fragmentManager = null;
+        loaderManager = null;
     }
 
     @UiThread