autofillError if extension is opened while page is loading

Hi,

I've found a problem with using bitwarden with Firefox browser.

### Prerequisities:

- login page is loading and do not load any fields now.

### How to reproduce:

- when login page is loading,
- and i open browser popup i can choose between credentials,
- when page loads i click one of credentials.

### What is now:
- error popup displays with `autofillError` text, because of not detected fields on page.

### Expected:
- fields are obviously on now loaded page, thus should be filled with credentials.

I bet this field detecting problem on page is caused by page scanning on popup opening event, which should be on credentials click event instead.
Additionally it will probably speed up opening main popup on very complex pages.

Reproduce the flow in the Firefox browser extension: open the browser popup while a login page is loading, choose credentials, and wait for the page fields to appear. Trace the popup and autofill flow from that interaction; done means credentials fill successfully without an autofillError when fields load after the popup opens.

Written by the indexing model from the issue text.