suppress warning for no donation solicited

fulcrm_shoppingcart.module

@@ -792,7 +792,9 @@ function fulcrm_shoppingcart_buy_eventbookingtype( $person, $eventbookingtype, $
 
 function fulcrm_shoppingcart_buy_donation_form_submit( $form, &$form_state ) {
     $product_id = $form_state[ 'fulcrm_shoppingcart' ][ 'product' ][ 'id' ];
-    $donation_amount = floatval( $form_state[ 'values' ][ 'donation_amount' ] );
+    $donation_amount = 0.0;
+    if ( array_key_exists( 'donation_amount', $form_state[ 'values' ] ) )
+        $donation_amount = floatval( $form_state[ 'values' ][ 'donation_amount' ] );
 
     if ( $donation_amount > 0.0 ) {
         $person_uuid = NULL;