restored EntitySelector preprocessor invocation

this was first introduced in c2deddb4d2e9e1ff2b1c2a8d4ac0697a56dc669c,
but was erroneously removed in 8c95e611c3df4dcf9e32b097d8ebb25cca020d2e

hopefully that's the only thing that was lost - clearly we need more
tests (indeed, this widget should live in a separate repo)

/cc @mrreynolds