Settings form 'Add another ID' error: Undefined variable 'accounts'

Problem/Motivation

When clicking the 'Add another ID' button in the settings form, an AJAX error is reported: 'Undefined variable: accounts in /(...)/google_analytics/src/Form/GoogleAnalyticsAdminSettingsForm.php on line 145'.

Proposed resolution

Attached patch assigns $accounts variable outside the $id_count === NULL check. Also, using the (string) cast for $accounts[$i] if it's not set reports an 'Undefined offset' notice if $accounts is empty (but $id_count is force-set to 1) - even with the null coalescing operator. Patch expands this usage to include an isset() check.