First fix the nullref.
How to fix a NullReferenceException error
https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/
Three steps to success:
- Identify what is null ← any other action taken before this step is WASTED TIME
- Identify why it is null
- Fix that
In the future if you post code, please ALWAYS USE CODE TAGS:
How to use code tags: Using code tags properly