Fixes the crash when the transient parent is NULL.

chrome/browser/ui/libgtk2ui/gtk2_util.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/libgtk2ui/gtk2_util.h"
 
 #include <gdk/gdk.h>
 #include <gdk/gdkx.h>
 #include <gtk/gtk.h>
 
@@ skipping 85 matching lines @@
   flags |= (state & GDK_CONTROL_MASK) ? ui::EF_CONTROL_DOWN : ui::EF_NONE;
   flags |= (state & GDK_SHIFT_MASK) ? ui::EF_SHIFT_DOWN : ui::EF_NONE;
   flags |= (state & GDK_MOD1_MASK) ? ui::EF_ALT_DOWN : ui::EF_NONE;
   flags |= (state & GDK_BUTTON1_MASK) ? ui::EF_LEFT_MOUSE_BUTTON : ui::EF_NONE;
   flags |=
       (state & GDK_BUTTON2_MASK) ? ui::EF_MIDDLE_MOUSE_BUTTON : ui::EF_NONE;
   flags |= (state & GDK_BUTTON3_MASK) ? ui::EF_RIGHT_MOUSE_BUTTON : ui::EF_NONE;
   return flags;
 }
 
-// Set |dialog| as transient for |parent|, which will keep it on top and center
-// it above |parent|.
 void SetGtkTransientForAura(GtkWidget* dialog, aura::Window* parent) {
+  if (!parent)
+    return;
+
   gtk_widget_realize(dialog);
   GdkWindow* gdk_window = gtk_widget_get_window(dialog);
 
   // TODO(erg): Check to make sure we're using X11 if wayland or some other
   // display server ever happens. Otherwise, this will crash.
   XSetTransientForHint(GDK_WINDOW_XDISPLAY(gdk_window),
                        GDK_WINDOW_XID(gdk_window),
                        parent->GetHost()->GetAcceleratedWidget());
 
   // We also set the |parent| as a property of |dialog|, so that we can unlink
   // the two later.
   g_object_set_data(G_OBJECT(dialog), kAuraTransientParent, parent);
 }
 
 aura::Window* GetAuraTransientParent(GtkWidget* dialog) {
   return reinterpret_cast<aura::Window*>(
       g_object_get_data(G_OBJECT(dialog), kAuraTransientParent));
 }
 
 void ClearAuraTransientParent(GtkWidget* dialog) {
   g_object_set_data(G_OBJECT(dialog), kAuraTransientParent, NULL);
 }
 
 }  // namespace libgtk2ui