because the Env is kept until we exit, which we get away with now. But we won't
get away with it if we set the parent window property of the InteractionHandler
belonging to the Env. Because we then end up trying to destroy the vcl dialog
well after all the vcl, etc infrastructure is torn down and crash on exit.

Here I'm assuming that its safe to use a WeakReference because we're just using
this to smuggle into the c-style callbacks a Reference to a Env currently
belonging to something else

Change-Id: I2d6b90ae23d5a24431dc49d4316bdc3194560403