See, for example, this partial stack-trace from openrdf-server for an HTTP DELETE operation on the federation repository 'testfed2':
The line in the code has recieved a null reference to the proxied repository to call `initializeInternal()` on, because it has been previously deleted. Here's the situation from the Console user's viewpoint:
I'm not sure what the best way to fix or mitigate this should be. Perhaps the Console and Workbench could have logic added that checks whether a repository requested to be dropped has any ProxyRepository referring to it. Then a warning with an opportunity to cancel the operation could be issued.