[spark] Add error message when sessionCatalog is null (#4161)

paimon-spark/paimon-spark-common/src/main/java/org/apache/paimon/spark/SparkGenericCatalog.java

@@ -66,6 +66,7 @@
 import static org.apache.paimon.options.CatalogOptions.WAREHOUSE;
 import static org.apache.paimon.spark.SparkCatalogOptions.CREATE_UNDERLYING_SESSION_CATALOG;
 import static org.apache.paimon.spark.SparkCatalogOptions.DEFAULT_DATABASE;
+import static org.apache.paimon.utils.Preconditions.checkNotNull;
 
 /* This file is based on source code from the Iceberg Project (http://iceberg.apache.org/), licensed by the Apache
  * Software Foundation (ASF) under the Apache License, Version 2.0. See the NOTICE file distributed with this work for
@@ -338,16 +339,24 @@ public void setDelegateCatalog(CatalogPlugin delegate) {
         }
     }
 
+    private CatalogPlugin getDelegateCatalog() {
+        checkNotNull(
+                sessionCatalog,
+                "Delegated SessionCatalog is missing, '%s' can only be used with 'spark_catalog'.",
+                SparkGenericCatalog.class.getName());
+        return sessionCatalog;
+    }
+
     private TableCatalog asTableCatalog() {
-        return (TableCatalog) sessionCatalog;
+        return (TableCatalog) getDelegateCatalog();
     }
 
     private SupportsNamespaces asNamespaceCatalog() {
-        return (SupportsNamespaces) sessionCatalog;
+        return (SupportsNamespaces) getDelegateCatalog();
     }
 
     private FunctionCatalog asFunctionCatalog() {
-        return (FunctionCatalog) sessionCatalog;
+        return (FunctionCatalog) getDelegateCatalog();
     }
 
     @Override