[BUGFIX] Configuration controller fatals with local-only extension

If the settings of a local-only extension get saved the configuration
controller fatals. This is because the result of the extension model
repository does not get checked, if it really is a model.

With this fix the instance is first checked and afterwards the category
is read from the model.

Resolves: #51902
Releases: 6.2
Change-Id: I543aa0960f3c2ba1877189b44e47aa7f86c81928
Reviewed-on: https://review.typo3.org/23738
Reviewed-by: Sebastian Michaelsen
Tested-by: Sebastian Michaelsen
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn