Validate method parameters when registering events. Fixes BUKKIT-3779

By: feildmaster <admin@feildmaster.com>

paper-api/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java

@@ -380,8 +380,8 @@ public class JavaPluginLoader implements PluginLoader {
         for (final Method method : methods) {
             final EventHandler eh = method.getAnnotation(EventHandler.class);
             if (eh == null) continue;
-            final Class<?> checkClass = method.getParameterTypes()[0];
-            if (!Event.class.isAssignableFrom(checkClass) || method.getParameterTypes().length != 1) {
+            final Class<?> checkClass;
+            if (method.getParameterTypes().length != 1 || !Event.class.isAssignableFrom(checkClass = method.getParameterTypes()[0])) {
                 plugin.getLogger().severe(plugin.getDescription().getFullName() + " attempted to register an invalid EventHandler method signature \"" + method.toGenericString() + "\" in " + listener.getClass());
                 continue;
             }