validator(value) {
        const acceptableValues = [ 'auto', 'top', 'bottom', 'above', 'below' ]
        return includes(acceptableValues, value)
      }
By default (`auto`), the menu will open below the control. If there is not
enough space, vue-treeselect will automatically flip the menu.
You can use one of other four options to force the menu to be always opened
to specified direction.
Acceptable values:
  - `"auto"`
  - `"below"`
  - `"bottom"`
  - `"above"`
  - `"top"`