FSE: PHP warning from lib/templates.php

[carolinan]

Describe the bug
With the current master (55bf177),
I am seeing the following PHP warning in the admin area under Appearance, Templates, in the Theme column:

Warning: Invalid argument supplied for foreach() in /var/www/html/wp-content/plugins/gutenberg-git/lib/templates.php on line 247

See 974906b#diff-70c8debe9dcb475ca0a1d26dc8078c0e816e0a5d783488280b5aa5aff956b5abR247

To reproduce
Steps to reproduce the behavior:

1. Activate the current master version of the plugin
2. In the WordPress admin, click on Appearance, Templates.
3. See error

You may need to test it on an install that had templates created/saved before the change with the theme column was made.

Expected behavior
There should not be a PHP warning

Editor version (please complete the following information):

• WordPress version: 5.6-RC1-49667
• If the Gutenberg plugin is installed, which version is it? Current master. The warning is not present in 9.4.1.

Desktop (please complete the following information):

• OS: Windows 10
• Browser Chrome Version 87.0.4280.66
  PHP version 7.4.11