fix(modpack): 修复本地模组包安装任务为空时的异常崩溃

- 添加空值检查避免在task为null时调用installModpack方法
- 防止因空指针异常导致的程序崩溃
- 确保只有在任务存在时才执行模组包安装流程

Author: ShirosakiMio

FCL/src/main/java/com/tungsten/fcl/ui/download/modpack/LocalModpackPage.java

@@ -165,7 +165,8 @@ protected void onInstall() {
                 task = ModpackInstaller.getModpackInstallTask(getContext(), profile, updateVersion, modpackFile, manifest, name);
             }
         }
-        ModpackInstaller.installModpack(getContext(), task, updateVersion != null);
+        if (task != null)
+            ModpackInstaller.installModpack(getContext(), task, updateVersion != null);
     }
 
     @Override