[release/3.6] 修复 backgroundType 为 null 时启动器崩溃的问题 (#4438)

#4437

Author: Glavo

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

@@ -198,6 +198,8 @@ public Decorator getDecorator() {
 
     private Background getBackground() {
         EnumBackgroundImage imageType = config().getBackgroundImageType();
+        if (imageType == null)
+            imageType = EnumBackgroundImage.DEFAULT;
 
         Image image = null;
         switch (imageType) {