I have installed the Composer-module for EPiServer on an EPiServer CMS 6 R2-site deployed at IIS 7.
I have configured the web.config according to the instructions.
I can view a page in 'Composer edit'-mode.

I get the following error when I try to access the Composer-plugin in Admin-mode (which is used to register Composer-blocks).

How do I solve this?:
    Server Error in '/' Application.
    Object reference not set to an instance of an object.
    Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
    
    Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
    
    Source Error:
    
    
    Line 60:                             <ItemTemplate>
    Line 61:                                 <nobr>  
    Line 62:                                     <asp:Label ID="Label1" runat="server"
    Line 63:                                         ToolTip ='<%#DataBinder.Eval(Container.DataItem, "Description")%>'
    Line 64:                                         Text = '<%# Utils.TrimDescription(DataBinder.Eval(Container.DataItem, "Description").ToString(),20).ToHtmlEncoded()%>'/>         
    
    
    Source File: c:\EPiServer\Sites\Skien\Dropit\Plugin\Extension\UI\Admin\Controls\FunctionTypeList.ascx    Line: 62
    
    Stack Trace:
    
    
    [NullReferenceException: Object reference not set to an instance of an object.]
       ASP.dropit_plugin_extension_ui_admin_controls_functiontypelist_ascx.__DataBinding__control19(Object sender, EventArgs e) in c:\EPiServer\Sites\Skien\Dropit\Plugin\Extension\UI\Admin\Controls\FunctionTypeList.ascx:62
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +303
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       System.Web.UI.WebControls.GridView.CreateRow(Int32 rowIndex, Int32 dataSourceIndex, DataControlRowType rowType, DataControlRowState rowState, Boolean dataBind, Object dataItem, DataControlField[] fields, TableRowCollection rows, PagedDataSource pagedDataSource) +295
       System.Web.UI.WebControls.GridView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding) +4780
       System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data) +95
       System.Web.UI.WebControls.GridView.PerformDataBinding(IEnumerable data) +19
       System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) +222
       System.Web.UI.WebControls.DataBoundControl.PerformSelect() +313
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       System.Web.UI.Control.DataBindChildren() +12659735
       System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +321
       Dropit.Extension.UI.Admin.Controls.FunctionTypeList.DataBind() +646
       System.Web.UI.Control.LoadRecursive() +71
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Control.LoadRecursive() +190
       System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3178
    
    
    Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17929