check if a plugin has a executor service (#1038)

* check if a plugin has an executor service

* checkstyle

proxy/src/main/java/com/velocitypowered/proxy/plugin/loader/VelocityPluginContainer.java

@@ -71,4 +71,8 @@ public class VelocityPluginContainer implements PluginContainer {
 
     return this.service;
   }
+
+  public boolean hasExecutorService() {
+    return this.service != null;
+  }
 }

proxy/src/main/java/com/velocitypowered/proxy/scheduler/VelocityScheduler.java

@@ -30,9 +30,12 @@ import com.velocitypowered.api.scheduler.ScheduledTask;
 import com.velocitypowered.api.scheduler.Scheduler;
 import com.velocitypowered.api.scheduler.TaskStatus;
 import com.velocitypowered.proxy.plugin.loader.VelocityPluginContainer;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.IdentityHashMap;
+import java.util.Iterator;
+import java.util.List;
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
@@ -116,17 +119,24 @@ public class VelocityScheduler implements Scheduler {
       task.cancel();
     }
     timerExecutionService.shutdown();
-    for (final PluginContainer container : this.pluginManager.getPlugins()) {
+    final List<PluginContainer> plugins = new ArrayList<>(this.pluginManager.getPlugins());
+    final Iterator<PluginContainer> pluginIterator = plugins.iterator();
+    while (pluginIterator.hasNext()) {
+      final PluginContainer container = pluginIterator.next();
       if (container instanceof VelocityPluginContainer) {
-        (container).getExecutorService().shutdown();
+        final VelocityPluginContainer pluginContainer = (VelocityPluginContainer) container;
+        if (pluginContainer.hasExecutorService()) {
+          container.getExecutorService().shutdown();
+        } else {
+          pluginIterator.remove();
+        }
+      } else {
+        pluginIterator.remove();
       }
     }
 
     boolean allShutdown = true;
-    for (final PluginContainer container : this.pluginManager.getPlugins()) {
-      if (!(container instanceof VelocityPluginContainer)) {
-        continue;
-      }
+    for (final PluginContainer container : plugins) {
       final String id = container.getDescription().getId();
       final ExecutorService service = (container).getExecutorService();