Crash when mapping unsat core to terms

[balasub]

The model below can be used to reproduce a NullReferenceException that happens with the following sequence of actions:

1. Load model
2. solve pm 1 Graph.conforms
3. ex 0 0 0

domain Graph
{
  Node ::= new (id: Integer).
  Edge ::= new (src: Integer, dst: Integer).

  edgeHasSrc ::= (src: Integer).
  edgeHasDst ::= (dst: Integer).

  path ::= (Integer, Integer).
  cycle ::= (Integer).

  badSrc ::= (src: Integer).
  badDst ::= (dst: Integer).

  edgeHasSrc(a) :- Edge(a, b), Node(a).
  edgeHasDst(b) :- Edge(a, b), Node(b).

  badSrc(a) :- Edge(a, b), no edgeHasSrc(a).
  badDst(b) :- Edge(a, b), no edgeHasDst(b).

  path(a, b) :- Edge(a, b).
  path(a, c) :- path(a, b), path(b, c).
  cycle(a) :- path(a, a).

  conforms no badSrc(_).
  conforms no badDst(_).

  enoughNodes :- count({ n | n is Node(_)}) > 4. 
  conforms enoughNodes.

  enoughEdges :- count({ e | e is Edge(_, _)}) = 3. 
  conforms enoughEdges.
  
  enoughCycles :- count({ c | c is cycle(_)}) = 4. 
  conforms enoughCycles.
}

partial model pm of Graph
[solver_RecursionBound = 5]
{
  Node(t).
  Node(u).
  Node(v).
  Node(w).
  Node(x).
  Edge(l, m).
  Edge(n, o).
  Edge(p, q).
  Edge(r, s).
  Edge(a, b).
  Edge(c, d).
}

Stack trace below.