With two extension windows open, delete confirmation opens in both windows

If I open two extensions windows in either Chrome or Firefox, delete confirmations open in both windows.

Steps to reproduce:
1. Open Firefox or Chrome with Bitwarden extension installed
2. Click on the extension icon
3. Click the 'Pop out to a new window' button twice to open two windows
4. Navigate to an existing entry (any type: login, secure note, etc)
5. Click the 'Edit' button
6. Click the 'Delete Item' button

Expected result:
The confirmation popup should only appear in the extension window in which the 'Delete Item' button was clicked.

Actual result:
The confirmation popup appears in both extension windows.

Bitwarden extension version: 1.26.3

Browser information:
- Firefox: 59.0.2
- Google Chrome: 66.0.3359.139

Computer information:
- Mac OSX 10.11.6

Reproduce the issue in two popped-out Bitwarden extension windows in Firefox or Chrome, following the listed steps through Delete Item. Trace the delete confirmation flow from that entry point and verify that the confirmation appears only in the window where the action was taken.

Written by the indexing model from the issue text.