修复无graphics报错问题

source/src/Graphics/Renderers/Renderer.ts

@@ -14,22 +14,34 @@ module es {
         public unload() { }
     
         protected beginRender(cam: ICamera) {
+            if (!Graphics.instance)
+                return;
+
             Graphics.instance.batcher.begin(cam);
         }
     
         protected endRender() {
+            if (!Graphics.instance)
+                return;
+
             Graphics.instance.batcher.end();
         }
     
         public abstract render(scene: Scene): void;
     
         protected renderAfterStateCheck(renderable: IRenderable, cam: ICamera) {
+            if (!Graphics.instance)
+                return;
+            
             renderable.render(Graphics.instance.batcher, cam);
         }
     
         protected debugRender(scene: Scene) {
-            es.Physics.debugDraw(2);
+            if (!Graphics.instance)
+                return;
             
+            es.Physics.debugDraw(2);
+
             for (let i = 0; i < scene.entities.count; i ++) {
                 let entity = scene.entities.buffer[i];
                 if (entity.enabled) {