index.html:151 Uncaught TypeError: Cannot read property 'addEventListener' of null code example

Example 1: Uncaught TypeError: Cannot read property 'addEventListener' of undefined

1)Check you placed .before the class
Example:
document.getElementsByClassName('.yourclassname') //.before class name is wrong
the code should be document.getElementsByClassName('yourclassname');
2)Check the spelling for addEventListener 
///dont feel embrassed because i always do these mistakes eventhough i used this property it 10000 times

Example 2: Cannot read property 'addEventListener' of null

if (document.body.contains(document.getElementById('someid'))) {
    // do what you need
}

// or 

if (!document.visibilityState) {
    // do what you need
}

// or 

if (!document.getElementsByClassName('.someclass')){
    // do what you need
}