Calls to GET api/Federation/members return 400 Bad Request

[drmathias]

Steps to reproduce:

1. Run Stratis.CirrusMinerD
2. Make a request to GET /api/Federation/members

This results in a 400 Bad Request which contains an error describing a NullReferenceException

{
  "errors": [
    {
      "status": 400,
      "message": "Object reference not set to an instance of an object.",
      "description": "System.NullReferenceException: Object reference not set to an instance of an object.\n   at Stratis.Bitcoin.Features.PoA.Voting.FederationController.GetMembers() in /Users/drmathias/projects/Stratis/StratisFullNode/src/Stratis.Bitcoin.Features.PoA/Voting/FederationController.cs:line 170"
    }
  ]
}