NPE in JcrResourceProviderFactory due to "impossible" null Repository reference

After heavy SCR-related activity, I get an NPE at JcrResourceProviderFactory.java:136, which is

session = repository.loginAdministrative(workspace);

and repository is null, which in theory cannot happen as that's an @Reference.

This looks similar to FELIX-3317, but reporting here as it affects Sling and is reproducible as follows:

1. Start the sling launchpad/builder

2. Install my test bundle from https://github.com/bdelacretaz/bundle-start-stop-test, which semi-randomly stops and restarts bundles to trigger SCR activity

3. When the test bundle says "Waiting up to 10000 msec to start next cycle" in the log, request / and check the logs for the NPE. Maybe set a breakpoint on that log message to better control the sequence

In my case I get the NPE after 3-4 cycles of the test stopping/restarting bundles.