Fix project bundle getting wiped in Designer

Steps to reproduce:
1. Switch to simple commit mode.
2. In Designer, have a couple of UI contributions in the bundle (some buttons for example).
3. In the buttons view, disable one of the custom ones.
4. Switch to another editor (Drawer Items, for example) as quickly as possible (before the loading end right after the previous point).
5. Notice how there are no longer any UI contributions. Checking the bundle, you'll see that it has been wiped.

There's no mechanism in Designer to remove more than 1 or 2 UI contributions from the bundle at the same time, but there is one to initialize the bundle. In this case, it seems that the bundle is getting re-initialized when a user tries to open another editor before the disable contribution is finished saving.
This might be a good opportunity to review how/when the Designer bundle should be initialized.