[Question] Dubbo proxy occurs NullPointerException, have no "dubbo" Decorator

### Question

When i use the shenyu to proxy dubbo service and then call the gateway interface.

java.lang.NullPointerException: null
at org.apache.shenyu.plugin.global.DefaultShenyuContextBuilder.build(DefaultShenyuContextBuilder.java:61) ~[shenyu-plugin-global-2.5.1.jar:2.5.1]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:

The decoratorMap just have http. When the DubboShenyuContextDecorator was added to this map?

maybe i miss some dependencies ... but i can't find it in document.

Start with org.apache.shenyu.plugin.global.DefaultShenyuContextBuilder at line 61 and inspect how decoratorMap is populated. Trace the referenced DubboShenyuContextDecorator and the documented dependencies for Dubbo proxying. Done means the cause of the missing decorator is identified and a Dubbo gateway call no longer produces this NullPointerException, with the required setup clear.

Written by the indexing model from the issue text.