magento-engcom/import-export-improvements#64: fix issue with Export Type UI

 - reset the entity type in the export form JS
 - this change was made so that if you choose an entity type, then choose the "Please Select" option the entity type gets reset and so now if you choose another type the form will actually be shown

Author: dmanners

app/code/Magento/ImportExport/view/adminhtml/templates/export/form/before.phtml

@@ -64,6 +64,7 @@ require([
                     this.modifyFilterGrid();
                 }
             } else {
+                this.previousGridEntity = '';
                 $('export_filter_container').hide();
             }
         }