JS help needed. !Cannot read property 'value' of null

Hi everyone.
I have created a basic program that tells you if the entered value is an even or odd number and it keeps showing this error message. Uncaught TypeError: Cannot read property ‘value’ of null on Line: 2. The program works but I would still like to know why this error message keeps showing and how I can fix it.

Thanks for the help.

Line 2 is trying to get the value property from number-input, and it has no value property, so you get the error.