Fix #5256: 修复使用 GIF 背景时调整背景不透明度导致启动器崩溃的问题 (#5262)

Author: Glavo

HMCL/src/main/java/org/jackhuang/hmcl/ui/decorator/DecoratorController.java

@@ -264,7 +264,7 @@ private Background getBackground() {
     private Background createBackgroundWithOpacity(Image image, int opacity) {
         if (opacity <= 0) {
             return new Background(new BackgroundFill(new Color(1, 1, 1, 0), CornerRadii.EMPTY, Insets.EMPTY));
-        } else if (opacity >= 100) {
+        } else if (opacity >= 100 || image.getPixelReader() == null) {
             return new Background(new BackgroundImage(
                     image,
                     BackgroundRepeat.NO_REPEAT,