better null check in default codegen (#13264)

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -6926,6 +6926,10 @@ protected void updateRequestBodyForArray(CodegenParameter codegenParameter, Sche
             final ArraySchema arraySchema = (ArraySchema) schema;
             Schema inner = getSchemaItems(arraySchema);
             CodegenProperty codegenProperty = fromProperty("property", arraySchema, false);
+            if (codegenProperty == null) {
+               throw new RuntimeException("CodegenProperty cannot be null. arraySchema for debugging: " + arraySchema);
+            }
+
             if (!addSchemaImportsFromV3SpecLocations) {
                 imports.add(codegenProperty.baseType);
             }
@@ -6943,6 +6947,10 @@ protected void updateRequestBodyForArray(CodegenParameter codegenParameter, Sche
                 innerCp = innerCp.items;
             }
 
+            if (mostInnerItem == null) {
+                throw new RuntimeException("mostInnerItem (codegen property of array item) cannot be null. " + arraySchema);
+            }
+
             if (StringUtils.isEmpty(bodyParameterName)) {
                 if (StringUtils.isEmpty(mostInnerItem.complexType)) {
                     codegenParameter.baseName = "request_body";