diff --git a/gluten-core/src/main/scala/org/apache/gluten/expression/AggregateFunctionsBuilder.scala b/gluten-core/src/main/scala/org/apache/gluten/expression/AggregateFunctionsBuilder.scala
index 6ac2c67eb0860..931fd07117643 100644
--- a/gluten-core/src/main/scala/org/apache/gluten/expression/AggregateFunctionsBuilder.scala
+++ b/gluten-core/src/main/scala/org/apache/gluten/expression/AggregateFunctionsBuilder.scala
@@ -29,32 +29,19 @@ object AggregateFunctionsBuilder {
     val functionMap = args.asInstanceOf[java.util.HashMap[String, java.lang.Long]]
 
     // First handle the custom aggregate functions
-    val (substraitAggFuncName, inputTypes) =
-      if (
-        ExpressionMappings.expressionExtensionTransformer.extensionExpressionsMapping.contains(
-          aggregateFunc.getClass)
-      ) {
-        val (substraitAggFuncName, inputTypes) =
-          ExpressionMappings.expressionExtensionTransformer.buildCustomAggregateFunction(
-            aggregateFunc)
-        assert(substraitAggFuncName.isDefined)
-        (substraitAggFuncName.get, inputTypes)
-      } else {
-        val substraitAggFuncName = getSubstraitFunctionName(aggregateFunc)
 
-        // Check whether each backend supports this aggregate function.
-        if (
-          !BackendsApiManager.getValidatorApiInstance.doExprValidate(
-            substraitAggFuncName,
-            aggregateFunc)
-        ) {
-          throw new GlutenNotSupportException(
-            s"Aggregate function not supported for $aggregateFunc.")
-        }
+    val substraitAggFuncName = getSubstraitFunctionName(aggregateFunc)
+
+    // Check whether each backend supports this aggregate function.
+    if (
+      !BackendsApiManager.getValidatorApiInstance.doExprValidate(
+        substraitAggFuncName,
+        aggregateFunc)
+    ) {
+      throw new GlutenNotSupportException(s"Aggregate function not supported for $aggregateFunc.")
+    }
 
-        val inputTypes: Seq[DataType] = aggregateFunc.children.map(child => child.dataType)
-        (substraitAggFuncName, inputTypes)
-      }
+    val inputTypes: Seq[DataType] = aggregateFunc.children.map(child => child.dataType)
 
     ExpressionBuilder.newScalarFunction(
       functionMap,
diff --git a/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionConverter.scala b/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionConverter.scala
index 3ca66b51897b0..fff914a45555a 100644
--- a/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionConverter.scala
+++ b/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionConverter.scala
@@ -155,8 +155,9 @@ object ExpressionConverter extends SQLConfHelper with Logging {
 
     expr match {
       case extendedExpr
-          if ExpressionMappings.expressionExtensionTransformer.extensionExpressionsMapping.contains(
-            extendedExpr.getClass) =>
+          if ExpressionMappings.expressionExtensionTransformer != null
+            && ExpressionMappings.expressionExtensionTransformer.extensionExpressionsMapping
+              .contains(extendedExpr.getClass) =>
         // Use extended expression transformer to replace custom expression first
         ExpressionMappings.expressionExtensionTransformer
           .replaceWithExtensionExpressionTransformer(substraitExprName, extendedExpr, attributeSeq)
diff --git a/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionMappings.scala b/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionMappings.scala
index e0628f11102d2..c07240efca7a7 100644
--- a/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionMappings.scala
+++ b/gluten-core/src/main/scala/org/apache/gluten/expression/ExpressionMappings.scala
@@ -338,8 +338,11 @@ object ExpressionMappings {
 
   def expressionsMap: Map[Class[_], String] = {
     val blacklist = GlutenConfig.getConf.expressionBlacklist
-    val supportedExprs = defaultExpressionsMap ++
-      expressionExtensionTransformer.extensionExpressionsMapping
+
+    var supportedExprs = defaultExpressionsMap
+    if (expressionExtensionTransformer != null) {
+      supportedExprs = supportedExprs ++ expressionExtensionTransformer.extensionExpressionsMapping
+    }
     if (blacklist.isEmpty) {
       supportedExprs
     } else {