Hi Hong ,
As I understand it, the problem is not that the file does not open.
There is a "null" in the files selected from the opened file. So if you're holding an attribute in the entity, it might be empty.
Or in one of the files in the folder you selected as directory, there is a value that the program sees as empty, it evaluates to "null". Unfortunately I am not experts on the subject. I hope it helps.
Regards
Ridvan