Always prefill event creation category if there is only the root category

[ThiefMaster]

Or even hide the category field altogether in this case?

[nop33]

You mean that event creation in the root category is allowed? No more just categories in root?

[ThiefMaster]

It is if there are no subcategories (has always been like this) - so same as any other category.

But showing the category field in the dialog is a bit pointless if there are no categories to choose from.