Notice on block config form

Problem/Motivation

Notice: Undefined index: pattern in Drupal\ui_patterns_blocks\PatternBlockFormAlter->getDefaultValue() (line 153 of modules/contrib/ui_patterns_blocks/src/PatternBlockFormAlter.php).

Steps to reproduce

Proposed resolution

We could just test for the key. But the underlying issue is that we're working with data from form fields that are not designd to be in a '#tree' => TRUE context and hence getting data that has extra nesting and cruft.

Better to fix that issue.

Options:

1. Add validation to form element to fix up data. Preferred.
2. Act at entity presave to fix up data.

Remaining tasks

User interface changes

API changes

Data model changes