Uncaught Error: Call to a member function get_type() on bool - BackinStock Notifier Wordpress Plugin

It's because of the following code, the error was being thrown.

/plugins/backinstocknotifier/backinstocknotifier.php

            $product = wc_get_product($post_id);  
            if($product->get_type() == 'variable') {
                // the rest of the code
            }

It doesn't check the post is the product or not, so when the post is not the product, the plugin will throw error.
So we can correct the code such as,

            $product = wc_get_product($post_id);  
            if(isset($product) && $product === false){return;} // exit the function if the post_id not corresponding to product
            if($product->get_type() == 'variable') {
                // the rest of the code
            }