fix: no-op when removing an already-removed glass

[ohxyz]

Problem

removeWindowlessGlass and removeDetachedGlass pass detachedGlassManager.removeDetachedGlass(id) straight into removeDetachedGlassElement, which dereferences detachedGlassEl.id (via removeGlassBackdrop).

detachedGlassManager.removeDetachedGlass(id) returns null when the id isn't found — which happens whenever the glass was already removed, e.g. closed via its built-in close action. Removing it again by a stale id then crashes:

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'id')

Repro (via react-bwin)

1. Open two windowless glasses.
2. Close one via its close button (runs the built-in action → removeWindowlessGlass directly; react-bwin's live-id set still holds the closed id).
3. Unmount the WindowProvider → cleanup calls removeWindowlessGlass(staleId, { animate: false }) → crash.

Fix

Guard the null return in both remove paths so removing an already-gone glass is a harmless no-op (returns Promise.resolve(null)), consistent with the manager already returning null for an unknown id.

Test plan

• Call removeWindowlessGlass(id) twice for the same glass — second call resolves to null, no throw.
• Same for removeDetachedGlass(id).
• react-bwin: open two windowless glasses, close one, unmount the provider — no error, no orphaned glass.