新增获取图片时的报错提示

pom.xml

@@ -6,7 +6,7 @@
 
     <groupId>cn.lunadeer</groupId>
     <artifactId>ColorfulMap</artifactId>
-    <version>2.3</version>
+    <version>2.4</version>
     <packaging>jar</packaging>
 
     <name>ColorfulMap</name>

src/main/java/cn/lunadeer/colorfulmap/generator/Multi.java

@@ -31,6 +31,10 @@ public class Multi {
             }
             URL _url = new URL(url);
             BufferedImage raw_image = ImageIO.read(_url);
+            if (raw_image == null) {
+                Notification.error(player, "无法读取有效图片，请检查图片地址是否正确或者更换图床");
+                return null;
+            }
             BufferedImage resized_image;
             if (scale != 1.0) {
                 resized_image = ImageTool.resize(raw_image, scale);