[FTS Configuration] Fixed an typo - it should have been assignment

web/pgadmin/browser/server_groups/servers/databases/schemas/fts_configurations/static/js/fts_configuration.js

@@ -288,7 +288,7 @@ define('pgadmin.node.fts_configuration', [
 
         if (self.grid) {
           self.grid.remove();
-          self.grid.null;
+          self.grid = null;
         }
         // Initialize a new Grid instance
         var grid = self.grid = new Backgrid.Grid({