(Root cause: java.lang-IllegalArgumentException: objectis null) What is the source of the problem ?

What is the source of the problem ?

(Root cause: java.lang-IllegalArgumentException: objectis null)

This is a very common problem when using a Katalon keyword on an object but the object is null.

Example:

WebUI.click(findTestObject('object_id'),...). In this case, please check the object_id is correct or not.
WebUI.click(null). Users forgot to put a test object as a variable instead of null here.

Thanks.