Based on my discussion with the developer, this issue could occur if you have to many assembly references for your project.

However, in my test you could click the Show Advanced Properties link and set the Control ID from the advance interface to achieve the same result.

Based on the screenshot you supplied, it seems you are using our WebGrid control, if you would like to create a treeview like structure from a single table, you could use the WebGrid self reference feature. The self reference feature also already support load on demand scenario as demonstrated in the sample SelfRef_LoadOnDemandChildRetrieval.aspx which available on the provided WebGrid sample.

In We Grid 7, you will need to set the WebGrid column ColumnType to custom in order to render HTML code. Here is the snippet for the Tick column:

<ISWebGrid:WebGridColumn Name="Tick" ColumnType="Custom" Caption=" " Width="30px">
</ISWebGrid:WebGridColumn>

My test also replicate the issue you are reporting. It seems the issue is caused by a bug and a bug report has been logged for this issue.

You could try using multiple div with fixed width and eval keyword to achieve your scenario. Here is the snippet the I come up during my test for this scenario:

<ISWebGrid:WebGridColumn Name="View_WebGridColumn" EditType="NoEdit"
    ColumnType="Template" Caption="Custom View" Width="200px">
    <CellTemplate>
        <div style="width:148px; float:left; line-height: 20px; height: 20px; overflow:hidden; border-right: solid 1px #add1ff; padding-left:2px;"><%# Eval("ProductName") %></div>
        <div style="width:48px; float:left; line-height: 20px; height:20px; padding-left:2px;"><%# Eval("UnitPrice") %></div>
    </CellTemplate>
    <CellStyle>
        <Padding Bottom="0px" Left="0px" Right="0px" Top="0px" />
    </CellStyle>
</ISWebGrid:WebGridColumn>

Set the div width so it will fill all the column width. We must also set the div height to match the column height and finally set the cell padding to 0.