Added check for null values on existsOptionInSelectControl

See #1720 .. thanks @SteveyPugs

src/extensions/filter-control/bootstrap-table-filter-control.js

@@ -39,7 +39,7 @@
     var existOptionInSelectControl = function (selectControl, value) {
         var options = selectControl.get(selectControl.length - 1).options;
         for (var i = 0; i < options.length; i++) {
-            if (options[i].value === value.toString()) {
+            if (!value || options[i].value === value.toString()) {
                 //The value is not valid to add
                 return false;
             }