Disable customize direct checkout button if no variation selected

Hi there,

I have created a customize direct checkout button beside add to cart in single product page.

This is the code:

//add direct checkout button 
function insertcart_addtocart_button_func() {
        // echo content.
		global $product;
		$pid = $product->get_id();
		$quicklink = WC()->cart->get_checkout_url();
        echo '<div class="button quickcheckout"><a href="'.$quicklink.'?add-to-cart='.$pid.'">'.esc_attr('Quick Checkout').'</a></div>';
}
add_action( 'woocommerce_after_add_to_cart_button', 'insertcart_addtocart_button_func' );

But I want to disable the direct checkout button (make it unclickable) if no variation was selected. May I know what should I add to this code?

Thank you in advance.

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