Overlay disappears when creating a modal during the hidden event of an existing modal

[dtognazzini]

I am opening a new modal in the event handler for the 'hidden' event of a closing modal. This works fine, however, there is a race with the overlay/backdrop whereby the new modal shows without the backdrop.

The race is the same as described in #30. The order of events:

1. existing modal is hidden
2. hidden event is fired
3. new modal is created

• new modal doesn't create overlay due to the check here. This check ensures there aren't two overlays created.

4. the timeout in hide from the existing modal removes the overlay (here).

PR coming shortly.

[thednp]

Great work, thanks for the contribution.