In our application, there is a very complicated logic that creates a bunch of form fields based on some data. It's very complicated, we hate that logic; but it exists, it works, and it is being used in production.
Somewhere in that logic, under a very specific case, a datepicker form field is created, connected, and then immediately disconnected. Don't ask me why, I said it is complicated.
This commit fixes a bug in OverlayMixin. The `connectedCallback` calls `_setupOverlayCtrl()` asynchronously, after a `Promise.then()`. Thus, by the time this setup function is called, the component has already been disconnected (and `disconnectedCallback()` has already been called).
The fix is simple: don't try to setup the overlay if the component is no longer connected.
ef7f16a51c