[gnome-software] trivial: Fix a critical warning when clicking the category groups

[Richard Hughes <rhughes src gnome org>]

commit 4dbaa4832d0d000f480dde0bb85974105e90ef49
Author: Richard Hughes <richard hughsie com>
Date:   Thu Jul 18 10:31:43 2019 +0100

    trivial: Fix a critical warning when clicking the category groups

 src/gs-app-tile.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/gs-app-tile.c b/src/gs-app-tile.c
index 0efd254d..f276ae2b 100644
--- a/src/gs-app-tile.c
+++ b/src/gs-app-tile.c
@@ -63,7 +63,7 @@ gs_app_tile_set_app (GsAppTile *self, GsApp *app)
        g_set_object (&priv->app, app);
 
        /* optional refresh */
-       if (klass->refresh != NULL) {
+       if (klass->refresh != NULL && priv->app != NULL) {
                g_signal_connect (app, "notify::state",
                                  G_CALLBACK (gs_app_tile_state_changed_cb), self);
                g_signal_connect (app, "notify::name",