Notice: Undefined index: base theme in backdrop_find_base_themes() (line 854 of theme.inc)

[klonos]

Description of the bug

I bumped into this while working on a PR for #5186.

I did a bit of research, and there are a few mentions of this error around d.org, but nothing conclusive, or people close the issue because they cannot reproduce (https://www.drupal.org/project/fusion/issues/916212 for example).

Steps To Reproduce

Pass the name of any theme that doesn't specify a base theme as the $key parameter in backdrop_find_base_themes():

$all_themes = list_themes();
$base_themes = backdrop_find_base_themes($all_themes, 'seven');
dpm($base_themes);

Actual behavior

Notice: Undefined index: base theme in backdrop_find_base_themes()
(line 854 of /app/docroot/core/includes/theme.inc).

Expected behavior

No notice. The function should still return the same result (array(NULL)).

[klonos]

PR up for review: backdrop/backdrop#3748

[klonos]

I'll need to fix the test failures before this is ready for review.