Create mapping without id should throw an error

[VolkerHartmann]

Describe the bug
I tried to create a mapping for a plugin.
Unfortunately the ID was accidently removed.
This results in a broken mapping.
It was also not possible to delete the mapping due to the missing identifier.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Add mapping scheme'
2. Select one plugin and add a mapping without defining an id.
3. Try to delete the mapping

Expected behavior
It shouldn't be possible to create a mapping without an id.