As the solution in the provided forum thread is quite old, it is possible that the provided control template has changed and that is the reason you're observing the aforementioned error.
I've taken the liberty of creating a sample project for you to have a look at based on the solution my colleague offered. However, it uses the new GroupRenderMode="Flat"
which leads to better performance as described in our Grouping Modes article
. Bear in mind that when using this render mode, you should target the GroupHeaderRow element instead of the GridViewGroupRow as I've done in the sample project.
Please have a look and let me know if this solution would for you.
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and share your thoughts.