What does the JavaScript exception "x is not a non-null object" refer to?

This exception indicates that a JavaScript object was expected but was not provided. This can happen when a variable is referenced without being defined, or when a parameter is expected but not provided. For more detailed information, please refer to the Mozilla Developer Network documentation (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_non-null_object).