org.apache.dubbo.rpc.support.MockInvoker#getInterface should not return null, otherwise there's no way to specify URL with mock protocol in reference bean.

```xml
<dubbo:reference id="demoService" check="false" interface="o.DemoService" url="mock://..."/>
```
throw new RpcException(RpcException.BIZ_EXCEPTION, t);
            }
        } else { 
            try {
                Invoker<T> invoker = getInvoker(mock);
                return invoker.invoke(invocation);
            } catch (Throwable t) {
                throw new RpcException("Failed to create mock implementation class " + mock, t);
            }
        }
    }
    public static Throwable getThrowable(String throwstr) {
        Throwable throwable = throwables.get(throwstr);
        if (throwable != null) {
            return throwable;
        }
        try {
            Throwable t;
            Class<?> bizException = ReflectUtils.forName(throwstr);
            Constructor<?> constructor;
            constructor = ReflectUtils.findConstructor(bizException, String.class);
            t = (Throwable) constructor.newInstance(new Object[]{"mocked exception for service degradation."});
            if (throwables.size() < 1000) {
                throwables.put(throwstr, t);
            }
            return t;
        } catch (Exception e) {
            throw new RpcException("mock throw error :" + throwstr + " argument error.", e);
        }
    }