The error causes the editor's content area not to render. 

The following script can be used as a temporary workaround:

<telerik:RadEditor runat="server" ID="RadEdior1" OnClientSelectionChange="OnClientSelectionChange">
</telerik:RadEditor>

<script type="text/javascript">
    function OnClientSelectionChange(editor, args) {
        if (!editor.getSelectedElement()) {
            editor.focusFirstText();
        }
    }
</script>