Fixed remove menu error.

src/main/java/run/halo/app/controller/admin/api/MenuController.java

@@ -79,6 +79,13 @@ public class MenuController {
     @DeleteMapping("{menuId:\\d+}")
     @ApiOperation("Deletes a menu")
     public MenuDTO deleteBy(@PathVariable("menuId") Integer menuId) {
+        List<Menu> menus = menuService.listByParentId(menuId);
+        if (null != menus && menus.size() > 0) {
+            menus.forEach(menu -> {
+                menu.setParentId(0);
+                menuService.update(menu);
+            });
+        }
         return new MenuDTO().convertFrom(menuService.removeById(menuId));
     }
 }

src/main/java/run/halo/app/repository/MenuRepository.java

@@ -4,6 +4,8 @@ import org.springframework.lang.NonNull;
 import run.halo.app.model.entity.Menu;
 import run.halo.app.repository.base.BaseRepository;
 
+import java.util.List;
+
 /**
  * Menu repository.
  *
@@ -14,4 +16,6 @@ public interface MenuRepository extends BaseRepository<Menu, Integer> {
     boolean existsByName(@NonNull String name);
 
     boolean existsByIdNotAndName(@NonNull Integer id, @NonNull String name);
+
+    List<Menu> findByParentId(@NonNull Integer id);
 }

src/main/java/run/halo/app/service/MenuService.java

@@ -44,4 +44,12 @@ public interface MenuService extends CrudService<Menu, Integer> {
      * @return a menu tree
      */
     List<MenuVO> listAsTree(Sort sort);
+
+    /**
+     * Lists menu by parent id.
+     *
+     * @param id id
+     * @return a list of menu
+     */
+    List<Menu> listByParentId(@NonNull Integer id);
 }

src/main/java/run/halo/app/service/impl/MenuServiceImpl.java

@@ -71,6 +71,13 @@ public class MenuServiceImpl extends AbstractCrudService<Menu, Integer> implemen
         return topLevelMenu.getChildren();
     }
 
+    @Override
+    public List<Menu> listByParentId(Integer id) {
+        Assert.notNull(id, "Menu parent id must not be null");
+
+        return menuRepository.findByParentId(id);
+    }
+
     @Override
     public Menu create(Menu menu) {
         nameMustNotExist(menu);