ReferenceConfigCache cannot recover in 2.7.0.

[haiyang1985]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.0
• Operating System version: mac
• Java version: 1.8

Steps to reproduce this issue

1. start the consumer without provider, and it will throw "No Provider" exception. Below is not the full of my code, simplify on the ReferenceConfig.

try{
  ReferenceConfigCache cache = ReferenceConfigCache.getCache();
  Object proxy = cache.get(new ReferenceConfig());
}catch (Throwable e){
  e.printStackTrace();
}

2. Start the provider
3. Always get null even the provider started.
   Object proxyNew = cache.get(config);

Pls. provide [GitHub address] to reproduce this issue.

Expected Result

We are able to get proxy after start up the provider.
Object proxyNew = cache.get(config);

Actual Result

What actually happens?
cache always return null even provider started.

I think, we have to return null once consumer startup failed.

public <T> T get(ReferenceConfig<T> referenceConfig) {
    String key = generator.generateKey(referenceConfig);

    ReferenceConfig<?> config = cache.get(key);
    if (config != null) {
        return (T) config.get();
    }

    T t = referenceConfig.get();
    if (null == t){
        return null;
    }else{
        cache.putIfAbsent(key, referenceConfig);
        return t;
    }
}

Otherwise, the consumer cannot recover, as it have been initialized. The solution of #2233 is not able to resolve the issue.

private void init() {
    if (initialized) {
        return;
    }
    initialized = true;
}

[haiyang1985]

My mistake, the solution of #2233 is able to solve the issue.