MediaTypeMap shared across threads, but cache is not thread-safe

     Caused by: javax.ws.rs.ProcessingException: java.lang.NullPointerException: Cannot invoke "java.util.Map.put(Object, Object)" because "cache" is null
    	at org.jboss.resteasy.client.jaxrs.internal.ClientResponse.readFrom(ClientResponse.java:251)
    	at org.jboss.resteasy.specimpl.BuiltResponse.readEntity(BuiltResponse.java:88)
    	at org.jboss.resteasy.specimpl.AbstractBuiltResponse.readEntity(AbstractBuiltResponse.java:256)
    	at org.jboss.resteasy.client.jaxrs.internal.ClientInvocation.extractResult(ClientInvocation.java:163)
    	at org.jboss.resteasy.client.jaxrs.internal.ClientInvocation.lambda$getResponseTypeExtractor$6(ClientInvocation.java:613)
    	at org.jboss.resteasy.client.jaxrs.internal.ClientInvocation.lambda$asyncSubmit$10(ClientInvocation.java:756)
    	at org.jboss.resteasy.client.jaxrs.engines.jetty.JettyClientEngine$2.complete(JettyClientEngine.java:198)
    	at org.jboss.resteasy.client.jaxrs.engines.jetty.JettyClientEngine$2.onSuccess(JettyClientEngine.java:184)
    	... 24 more
    Caused by: java.lang.NullPointerException: Cannot invoke "java.util.Map.put(Object, Object)" because "cache" is null
    	at org.jboss.resteasy.core.MediaTypeMap.getPossible(MediaTypeMap.java:584)
    	at org.jboss.resteasy.core.providerfactory.ResteasyProviderFactoryImpl.resolveMessageBodyReader(ResteasyProviderFactoryImpl.java:576)
    	at org.jboss.resteasy.core.providerfactory.ResteasyProviderFactoryImpl.getClientMessageBodyReader(ResteasyProviderFactoryImpl.java:568)
    	at org.jboss.resteasy.core.interception.jaxrs.ClientReaderInterceptorContext.resolveReader(ClientReaderInterceptorContext.java:54)
    	at org.jboss.resteasy.core.interception.jaxrs.AbstractReaderInterceptorContext.getReader(AbstractReaderInterceptorContext.java:130)
    	at org.jboss.resteasy.core.interception.jaxrs.AbstractReaderInterceptorContext.proceed(AbstractReaderInterceptorContext.java:75)
    	at org.jboss.resteasy.client.jaxrs.internal.ClientResponse.readFrom(ClientResponse.java:214)

      if (useCache) {
         // don't care about variable volatility.  Really rare to add entries post boot
         Map<CachedMediaTypeAndClass, List<T>> cache = classCache;
         if (classCache == null) {
            cache = new HashMap<>();
            classCache = cache;
         }
         cache.put(cacheEntry, cached);
      }