Hi, when you post code to the forums, please use the code tags. There is information about that here:
Null reference exceptions are very common, so it would be a good idea to take a look at this thread:
https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/
Remember that the error message will tell you which line the error occurs on. In your case, it reports line 20. The only reference on this line is to currentTimeText, so you likely did not set this reference in the inspector.