_previousElement (elem) {
    const dom = this.dom
    // noinspection FallthroughInSwitchStatementJS
    switch (elem) {
      case dom.value:
        if (this.fieldEditable) {
          return dom.field
        }
      // intentional fall through
      case dom.field:
        if (this._hasChilds()) {
          return dom.expand
        }
      // intentional fall through
      case dom.expand:
        return dom.menu
      case dom.menu:
        if (dom.drag) {
          return dom.drag
        }
      // intentional fall through
      default:
        return null
    }
  }
Get the next element which can have focus.
@param {Element} elem
@return {Element | null} nextElem
@private