bugfix in the Java binding

src/runtime/java/jpgf.c

@@ -680,6 +680,8 @@ jpgf_literal_callback_fn(PgfLiteralCallback* self,
 
 	jstring jsentence = gu2j_string(env, sentence);
 	jobject result = (*env)->CallObjectMethod(env, callback->jcallback, callback->match_methodId, lin_idx, jsentence, *poffset);
+	if (result == NULL)
+		return NULL;
 
 	jclass result_class = (*env)->GetObjectClass(env, result);