In our UI for ASP.NET Core R3 2020 (2020.3.915) release, the Column menu message of unsticking a column is "null".
This bug will be resolved in our next official release.
In the meantime, as a workaround, manually set the Unstick Column menu message:
.ColumnMenu(c => c.Messages(m => m.Unstick(
The Bug in question has been fixed with the service pack following the R3 of 2020 - v2020.3.1021 Here is the link to the item: