Add null-check for customTypesMapping

Author: kobylynskyi

src/main/java/com/kobylynskyi/graphql/codegen/model/MappingConfig.java

@@ -21,6 +21,9 @@ public class MappingConfig {
     private String modelNameSuffix;
 
     public void putCustomTypeMappingIfAbsent(String from, String to) {
+        if (customTypesMapping == null) {
+            customTypesMapping = new HashMap<>();
+        }
         if (!customTypesMapping.containsKey(from)) {
             customTypesMapping.put(from, to);
         }

src/test/java/com/kobylynskyi/graphql/codegen/GraphqlCodegenTest.java

@@ -110,6 +110,19 @@ void generate_NoCustomMappings() throws Exception {
         assertThat(Utils.getFileContent(eventFile.getPath()), StringContains.containsString("String createdDateTime;"));
     }
 
+    @Test
+    void generate_NullCustomMappings() throws Exception {
+        mappingConfig.setCustomTypesMapping(null);
+        generator.generate();
+
+        File[] files = Objects.requireNonNull(outputJavaClassesDir.listFiles());
+        File eventFile = Arrays.stream(files)
+                .filter(file -> file.getName().equalsIgnoreCase("Event.java"))
+                .findFirst().orElseThrow(FileNotFoundException::new);
+
+        assertThat(Utils.getFileContent(eventFile.getPath()), StringContains.containsString("String createdDateTime;"));
+    }
+
     @Test
     void generate_NoPackage() throws Exception {
         mappingConfig.setPackageName(null);