fix image loading error: when an image fails to be loaded and initiated correctly, we should Assert the error in debug Mode and handle this situation by workaround in release Mode

Runtime/ui/painting/image.cs

             this._isDynamic = isDynamic;
         }
 
+        public bool valid {
+            get { return this._texture != null; }
+        }
+
         public int width {
             get { return this._texture != null ? this._texture.width : 0; }
         }

Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_impl.cs

         }
 
         void _drawImage(Image image, uiOffset offset, uiPaint paint) {
-            D.assert(image != null);
+            D.assert(image != null && image.valid);
+
+            if (image == null || !image.valid) {
+                return;
+            }
 
             this._drawImageRect(image,
                 null,
         }
 
         void _drawImageRect(Image image, uiRect? src, uiRect dst, uiPaint paint) {
-            D.assert(image != null);
+            D.assert(image != null && image.valid);
+
+            if (image == null || !image.valid) {
+                return;
+            }
 
             if (src == null) {
                 src = uiRectHelper.one;
         }
 
         void _drawImageNine(Image image, uiRect? src, uiRect center, uiRect dst, uiPaint paint) {
-            D.assert(image != null);
+            D.assert(image != null && image.valid);
+
+            if (image == null || !image.valid) {
+                return;
+            }
 
             var scaleX = 1f / image.width;
             var scaleY = 1f / image.height;