Don't crash if crm-submit-buttons element does not exist (eg. on webform_civicrm)

Author: mattwire

Metadata/js/omnipay_PaypalRest.js

@@ -91,7 +91,10 @@
           }
 
           document.getElementById('paypal-button-container').style.visibility = "hidden";
-          document.getElementById('crm-submit-buttons').style.display = 'block';
+          var crmSubmitButtons = document.getElementById('crm-submit-buttons');
+          if (crmSubmitButtons) {
+            crmSubmitButtons.style.display = 'block';
+          }
           document.getElementById('PayerID').value = data['payerID'];
           document.getElementById('payment_token').value = paymentToken;
           form.submit();