SPIGOT-4500: callSyncMethod broken

src/main/java/org/bukkit/craftbukkit/scheduler/CraftScheduler.java

@@ -437,7 +437,7 @@ public class CraftScheduler implements BukkitScheduler {
     private static void validate(final Plugin plugin, final Object task) {
         Validate.notNull(plugin, "Plugin cannot be null");
         Validate.notNull(task, "Task cannot be null");
-        Validate.isTrue(task instanceof Runnable || task instanceof Consumer, "Task must be Runnable or Consumer");
+        Validate.isTrue(task instanceof Runnable || task instanceof Consumer || task instanceof Callable, "Task must be Runnable, Consumer, or Callable");
         if (!plugin.isEnabled()) {
             throw new IllegalPluginAccessException("Plugin attempted to register task while disabled");
         }

src/main/java/org/bukkit/craftbukkit/scheduler/CraftTask.java

@@ -46,7 +46,7 @@ class CraftTask implements BukkitTask, Runnable {
             this.cTask = (Consumer<BukkitTask>) task;
             this.rTask = null;
         } else if (task == null) {
-            // Head task
+            // Head or Future task
             this.rTask = null;
             this.cTask = null;
         } else {