fix crash in NotificationsFragment when Preferences are first accessed

fixes #797

app/src/main/java/com/keylesspalace/tusky/adapter/NotificationsAdapter.java

@@ -233,6 +233,10 @@ public class NotificationsAdapter extends RecyclerView.Adapter {
         mediaPreviewEnabled = enabled;
     }
 
+    public boolean isMediaPreviewEnabled() {
+        return mediaPreviewEnabled;
+    }
+
     public interface NotificationActionListener {
         void onViewAccount(String id);
 

app/src/main/java/com/keylesspalace/tusky/fragment/NotificationsFragment.java

@@ -522,8 +522,10 @@ public class NotificationsFragment extends SFragment implements
             }
             case "mediaPreviewEnabled": {
                 boolean enabled = sharedPreferences.getBoolean("mediaPreviewEnabled", true);
+                if (enabled != adapter.isMediaPreviewEnabled()) {
                     adapter.setMediaPreviewEnabled(enabled);
                     fullyRefresh();
+                }
                 break;
             }
         }