[ISSUE #79] Fix CachedschemaRegistryClient.deleteSchema execution failed

Co-authored-by: xiaoyi <xiaojian.sxj@alibaba-inc.com>
diff --git a/client/src/main/java/org/apache/rocketmq/schema/registry/client/CachedSchemaRegistryClient.java b/client/src/main/java/org/apache/rocketmq/schema/registry/client/CachedSchemaRegistryClient.java
index 8e87666..d884492 100644
--- a/client/src/main/java/org/apache/rocketmq/schema/registry/client/CachedSchemaRegistryClient.java
+++ b/client/src/main/java/org/apache/rocketmq/schema/registry/client/CachedSchemaRegistryClient.java
@@ -131,7 +131,10 @@
         String subjectFullName = String.format("%s/%s/%s", cluster, tenant, subject);
         schemaCacheBySubject.invalidate(subjectFullName);
         schemaCacheBySubjectAndVersion.invalidate(new SubjectAndVersion(cluster, tenant, subject, version));
-        subjectToVersion.get(subjectFullName).remove(version);
+
+        if (subjectToSchema.get(subjectFullName) != null) {
+            subjectToVersion.get(subjectFullName).remove(version);
+        }
 
         return restService.deleteSchema(cluster, tenant, subject, version);
     }