The number used as prefix for element created in a diagram on a root Namespace is always `0`

[gcoutable]

Steps to reproduce

1. Create a SysMLv2 project
2. Disable the Hide Root Namespaces explorer filter to display root Namespaces
3. Create a General View diagram on a root Namespace
4. Create two PartUsage in the General View

Expected behavior

The number used as prefix for both PartUsage labels starts at 1 and increments with each new PartUsage.

Actual behavior

The number used as prefix for both PartUsage labels is 0