Increment by one the default child element creation number

STR:

go on a page element with some text lines (let's say 10 text line exist)
create a new text line using normal annotation (no batch annotation)
the new text line has the same number as the last pre-existing text line (so still 10) 🐛
create a new text line using normal annotation (no batch annotation)
that new text line increments properly to 11