My Button Default Click Is Not Working In WordPress Using Javascript Code


I have added 2 buttons in my page and I have added the default button click code using javascript but it is not working.

This is my HTML code:

<button class="tablink" onclick="openPage('inside', this, '#0077c0')" id="defaultOpen">Inside</button> <button class="tablink" style="left:40px;" onclick="openPage('outside', this, '#0077c0')" >Outside</button> 

This is JS code added in functions.php:

function wp_head_custom_mm(){ ?>  // Get the element with id="defaultOpen" and click on it document.getElementById("defaultOpen").click();  </script>     <?php         } add_action('wp_head', 'wp_head_custom_mm'); 

But it is not working, my default button is not clicked.

Error:

Uncaught TypeError: Cannot read property ‘click’ of null

I want to button with id defaultOpen should be clicked when the website opens.

Any help is much appreciated.