Bug 1613130 [Linux] Use GtkWindow as a fallback if a particular GtkWidget is missing, r=jhorak

Differential Revision: https://phabricator.services.mozilla.com/D61633

--HG--
extra : moz-landing-system : lando

widget/gtk/WidgetStyleCache.cpp

@@ -25,6 +25,7 @@ static GtkStyleContext* GetCssNodeStyleInternal(WidgetNodeType aNodeType);
 
 static GtkWidget* CreateWindowWidget() {
   GtkWidget* widget = gtk_window_new(GTK_WINDOW_POPUP);
+  MOZ_RELEASE_ASSERT(widget, "We're missing GtkWindow widget!");
   gtk_widget_set_name(widget, "MozillaGtkWidget");
   return widget;
 }
@@ -757,8 +758,12 @@ GtkWidget* GetWidget(WidgetNodeType aAppearance) {
   if (!widget) {
     widget = CreateWidget(aAppearance);
     // Some widgets (MOZ_GTK_COMBOBOX_SEPARATOR for instance) may not be
-    // available or implemented.
-    if (!widget) return nullptr;
+    // available or implemented. Use GtkWindow as a fallback to avoid
+    // potential crashes.
+    if (!widget) {
+      NS_WARNING(nsPrintfCString("Missing GtkWidget %d\n", aAppearance).get());
+      widget = CreateWindowWidget();
+    }
     // In GTK versions prior to 3.18, automatic invalidation of style contexts
     // for widgets was delayed until the next resize event.  Gecko however,
     // typically uses the style context before the resize event runs and so an