[gnome-software] trivial: Fix warning when calling gs_utils_widget_set_css_simple(NULL)

[Richard Hughes <rhughes src gnome org>]

commit e2af78378bfe72b60ba6a0afc72c510f5a6ab93c
Author: Richard Hughes <richard hughsie com>
Date:   Fri Aug 5 13:03:18 2016 +0100

    trivial: Fix warning when calling gs_utils_widget_set_css_simple(NULL)

 src/gs-common.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)
---
diff --git a/src/gs-common.c b/src/gs-common.c
index 5fb5371..cd41c86 100644
--- a/src/gs-common.c
+++ b/src/gs-common.c
@@ -658,9 +658,16 @@ void
 gs_utils_widget_set_css_simple (GtkWidget *widget, const gchar *css)
 {
        g_autofree gchar *class_name = NULL;
-       g_autoptr(GString) str = g_string_sized_new (1024);
+       g_autoptr(GString) str = NULL;
 
+       /* remove custom class if NULL */
        class_name = g_strdup_printf ("themed-widget_%p", widget);
+       if (css == NULL) {
+               GtkStyleContext *context = gtk_widget_get_style_context (widget);
+               gtk_style_context_remove_class (context, class_name);
+               return;
+       }
+       str = g_string_sized_new (1024);
        g_string_append_printf (str, ".%s {\n", class_name);
        g_string_append_printf (str, "%s\n", css);
        g_string_append (str, "}");