dropzone.js is giving error "Invalid dropzone element" on page without a dropzone

With pure JS you can try this:

if (document.getElementById('DropzoneElementId')) {
  var myDropzone = new Dropzone("div#DropzoneElementId", { url: "/file/post"});
  // other code here
}

or if you use jQuery:

if ($('#DropzoneElementId').length) {
  $("div#DropzoneElementId").dropzone({ url: "/file/post" });
  // other code here
}