Be resilient in case there is no device manager

It's possible to run Clutter with the 'null' input backend, which means
that clutter_device_manager_get_default() may return NULL. In the future
we may add a default dummy device manager, but right now it's safer to
just add a simple NULL check in the places where we ask for the device
manager.

clutter/clutter-main.c

@@ -2981,6 +2981,9 @@ clutter_grab_pointer_for_device (ClutterActor *actor,
     }
 
   manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return;
+
   dev = clutter_device_manager_get_device (manager, id_);
   if (dev == NULL)
     return;
@@ -3025,6 +3028,9 @@ clutter_ungrab_pointer_for_device (gint id_)
   ClutterInputDevice *device;
 
   manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return;
+
   device = clutter_device_manager_get_device (manager, id_);
   if (device != NULL)
     clutter_input_device_ungrab (device);
@@ -3293,6 +3299,8 @@ clutter_get_input_device_for_id (gint id_)
   ClutterDeviceManager *manager;
 
   manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return NULL;
 
   return clutter_device_manager_get_device (manager, id_);
 }

clutter/gdk/clutter-event-gdk.c

@@ -78,9 +78,12 @@ clutter_gdk_handle_event (GdkEvent *gdk_event)
   if (gdk_event->any.window == NULL)
     return GDK_FILTER_CONTINUE;
 
+  device_manager = clutter_device_manager_get_default ();
+  if (G_UNLIKELY (device_manager == NULL))
+    return GDK_FILTER_CONTINUE;
+
   backend_gdk = CLUTTER_BACKEND_GDK (backend);
   stage = clutter_gdk_get_stage_from_window (gdk_event->any.window);
-  device_manager = clutter_device_manager_get_default ();
 
   gdk_device = gdk_event_get_device (gdk_event);
   if (gdk_device != NULL)

clutter/osx/clutter-event-osx.c

@@ -306,14 +306,19 @@ clutter_event_osx_translate (NSEvent *nsevent,
                              ClutterEvent *event)
 {
   ClutterDeviceManagerOSX *manager_osx;
+  ClutterDeviceManager *manager;
   ClutterStageOSX *stage_osx;
   ClutterStageWindow *impl;
   ClutterStage *stage;
 
+  manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return FALSE;
+
   stage = event->any.stage;
   impl = _clutter_stage_get_window (event->any.stage);
   stage_osx = CLUTTER_STAGE_OSX (impl);
-  manager_osx = CLUTTER_DEVICE_MANAGER_OSX (clutter_device_manager_get_default ());
+  manager_osx = CLUTTER_DEVICE_MANAGER_OSX (manager);
 
   event->any.time = [nsevent clutterTime];
 

clutter/win32/clutter-event-win32.c

@@ -375,6 +375,9 @@ clutter_win32_handle_event (const MSG *msg)
     return TRUE;
 
   manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return FALSE;
+
   core_pointer =
     clutter_device_manager_get_core_device (manager, CLUTTER_POINTER_DEVICE);
   core_keyboard =

clutter/x11/clutter-backend-x11.c

@@ -1185,6 +1185,8 @@ clutter_x11_get_input_devices (void)
   ClutterDeviceManager *manager;
 
   manager = clutter_device_manager_get_default ();
+  if (manager == NULL)
+    return NULL;
 
   return clutter_device_manager_peek_devices (manager);
 }

clutter/x11/clutter-stage-x11.c

@@ -652,12 +652,16 @@ clutter_stage_x11_realize (ClutterStageWindow *stage_window)
    * the event mask we passed to XSelectInput as the template
    */
   device_manager = clutter_device_manager_get_default ();
+  if (G_UNLIKELY (device_manager != NULL))
+    {
       _clutter_device_manager_select_stage_events (device_manager,
                                                    stage_cogl->wrapper,
                                                    event_flags);
 
       g_signal_connect (device_manager, "device-added",
-                    G_CALLBACK (stage_events_device_added), stage_window);
+                        G_CALLBACK (stage_events_device_added),
+                        stage_window);
+    }
 
   clutter_stage_x11_fix_window_size (stage_x11,
                                      stage_x11->xwin_width,