The Template that was previously selected could have been modified
This message occurs if you have opened the Templates Manager from a GridTemplate property, then either changed the Template's properties, re-scanned it, or deleted it, and finally tried to close the Templates Manager without re-selecting a Template (ie. clicking on the x button at the top-right of the Templates Manager, or using ALT+F4).
Trying to do this is akin to clicking on the Cancel button which is disabled under those conditions. The reason is that clicking on the Cancel button tells Q++ that there has been no changes to the GridTemplate property, which may not be the case if you have modified its properties, re-scanned it or even deleted it.
Forcing you to select a Template (using the OK button) ensures that Q++ updates all the properties that depend on the internal details of the GridTemplate property.