无法获取未定义或空引用的属性“内容窗口”

我在引导模式中有一个Ajax编辑器。第一次打开模态时，它运行良好，下一次工具栏不再存在，它会抛出此错误。知道会发生什么吗？

谢谢

Unhandled exception at line 84, column 9 in http://localhost:53818/ScriptResource.axd?d=QnqJUlWX76vy-voPv0SOyeWuqjSyItje2m_RwWfpN9Dp9KL-kVEuy2eIP-IxAD4LMlqvG3esYD48cWHYJvzAF9a1YRm5XmxfCzWi0PaDVAfNV8mz4aOOLZVDVbMrKXACgpSJB28TI_zeX-HTr-9ysw2&t=ffffffffda74082d

在 ScriptResource.axd 中抛出错误 - this._doc = this.get_element（）.contentWindow.document;

 _initIframe: function(value) {
    if (!Sys.Extended.UI.HTMLEditor.isIE) {
        this._savedValue = value;
        this._absAndFixedParents = this._getAbsAndFixedParents();
    }
    var str = Sys.Extended.UI.HTMLEditor.Trim(this._prepareContent(value));
    this._doc = this.get_element().contentWindow.document;
    if (!Sys.Extended.UI.HTMLEditor.isIE) {
        this._doc.designMode = "on";
    }

这是模态

 <div id="modalEventsEditor" class="modal">
    <div class="modal-dialog">
        <div class="modal-content">

            <asp:UpdatePanel ID="EventsUpdatePanel" runat="server">
                <ContentTemplate>

                    <div class="modal-header">
                        <button class="close" aria-hidden="true" type="button" data-dismiss="modal">×</button>
                        <h4 class="modal-title">Events</h4>
                    </div>
                    <div class="modal-body">

                        <div role="form">

                            <fieldset>
                                <div id="eventDateDiv" class="form-group">
                                    <label id="eventDateLabel" for="inputEventDate">Date</label>
                                    <asp:TextBox ID="inputEventDate" CssClass="form-control" placeholder="The Date of the event" runat="server"></asp:TextBox>
                                    <ajaxToolkit:CalendarExtender runat="server" TargetControlID="inputEventDate"></ajaxToolkit:CalendarExtender>
                                </div>
                                <div id="eventTimeDiv" class="form-group">
                                    <label id="eventTimeLabel" for="inputEventTime">Time</label>
                                    <input id="inputEventTime" class="form-control" runat="server" type="text" placeholder="The Time of the event" />
                                </div>
                                <div id="eventAuthorDiv" class="form-group">
                                    <label id="eventAuthorLabel" for="inputEventAuthor">Author</label>
                                    <input id="inputEventAuthor" runat="server" class="form-control" type="text" placeholder="The Author of this notice" />
                                </div>
                                <div id="eventTitleDiv" class="form-group">
                                    <label id="eventTitleLabel" for="inputEventTitle">Title</label>
                                    <input id="inputEventTitle" runat="server" class="form-control" type="text" placeholder="The Title of this notice" />
                                </div>

                            </fieldset>

                        </div>
                        <div id="editorContainer">
                            <cc1:Editor ID="Editor1" runat="server" />
                        </div>

                    </div>

                    <div class="modal-footer">
                        <button class="btn btn-default" type="button" data-dismiss="modal">Close</button>
                        <button id="modalEventsEditorButton" runat="server" class="btn btn-primary" type="button">Save changes</button>
                    </div>
                </ContentTemplate>
            </asp:UpdatePanel>
        </div>
    </div>
</div>