Despite the fact that keyboard support is not supported with RadEditor, JavaScript error should not be thrown in such a situation.

You can workaround this by following this example code to override the method related to the error:

<telerik:RadEditor ID="RadEditor1" runat="server" EnableAriaSupport="true">
</telerik:RadEditor>

<script type="text/javascript">
    Telerik.Web.UI.EditorDropDown.prototype.hide = function () {
        if (this._popupBehavior) this._popupBehavior.hide();
        this._popupVisible = false;

        //Notify controller that tooltip was hidden
        this._getPopupVisibilityController().notifyPopupClosed(this);

        //Notify listeners for the dropdown being hidden
        this.raiseEvent("hide");

        if (this._popupBehavior && this.get_enableAriaSupport()) this._popupElement.setAttribute("aria-hidden", "true");
    };
</script>