我如何解决：对象引用没有设置为对象的实例？

Question

我收到错误消息＆＃39;对象引用未设置为对象的实例＆＃39;我的问题是：

你能解释一下这个决议，好像我是一个5岁的人。

ASCX

<asp:GridView ID="gvStaffStatus" runat="server"     
DataKeyNames="ID_NUM"
OnRowDataBound="gvStaffStatus_RowDataBound">      
    <Columns>       
    <asp:TemplateField ItemStyle-VerticalAlign="Top" HeaderText="Member" SortExpression="Member">
        <ItemTemplate>
            <asp:Image ID="imgStatus" Width="20px" runat="server" ImageUrl="~/Portlets/CUS/ICS/Portlet.KeepingTabs/available.png" />
            <asp:Label ID="lblStatus" runat="server"></asp:Label>
            <asp:DropDownList ID="ddlStatus" runat="server" OnSelectedIndexChanged="DdlStatus_SelectedIndexChange" AutoPostBack="true" Visible='<%# SetVisible(Eval("ID_NUM").ToString()) %>'>
                <asp:ListItem Selected ="True" Text="Available" Value="1" />
                <asp:ListItem Text="Busy" Value="2" />
                <asp:ListItem Text="Unavailable" Value="3" />
                <asp:ListItem Text="Off" Value="0" />
            </asp:DropDownList>
            <asp:Label runat="server" Text='<%# Bind("Member") %>' ID="lblMember"></asp:Label><br />
            <asp:DropDownList ID="ddlLocation" runat="server" AutoPostBack="true" Visible='<%# SetVisible(Eval("ID_NUM").ToString()) %>'>
                <asp:ListItem Selected="True" Text="My Office" Value="MYOFF" />
            </asp:DropDownList>
            <asp:TextBox Width="300px" ID="tbNote" runat="server" Visible='<%# SetVisible(Eval("ID_NUM").ToString()) %>'></asp:TextBox>
        </ItemTemplate>
    </asp:TemplateField>
    </Columns> 
</asp:GridView>

这一行似乎正在打破：

string status = ddlStatus.SelectedValue;

这些控件都在GridView中的单个TemplateField内的单个ItemTemplate内部

以下代码全部在我的OnInit方法中（但我也在自定义方法中尝试过）：

        DropDownList ddlStatus = (DropDownList)gvStaffStatus.FindControl("ddlStatus");
        Image imgStatus = (Image)gvStaffStatus.FindControl("imgStatus");

        string status = ddlStatus.SelectedValue;

        switch (status)
        {
            case "Available":
                imgStatus.ImageUrl = "~/Portlets/CUS/ICS/Portlet.KeepingTabs/available.png";
                break;
            case "Busy":
                imgStatus.ImageUrl = "~/Portlets/CUS/ICS/Portlet.KeepingTabs/busy.png";
                break;
            case "Unavailable":
                imgStatus.ImageUrl = "~/Portlets/CUS/ICS/Portlet.KeepingTabs/unavailable.png";
                break;
            default:
                imgStatus.ImageUrl = "~/Portlets/CUS/ICS/Portlet.KeepingTabs/available.png";
                break;
        }