Probably incorrect array check.

[sergekiryakov]

yii2-migration-utility/assets/AppUtility.php

Line 105 in 5b89422

$this->string .= (strlen($this->array['defaultValue']) == 0) ? '' : " DEFAULT " . $this->array['defaultValue']['expression'] . " ";

strlen() returns params error

[jwn81]

This seems to be a Bug and can be fixed in the line you referenced by correcting the line into this one:
$this->string .= (strlen($this->array['defaultValue']['expression']) == 0) ? '' : " DEFAULT " . $this->array['defaultValue']['expression'] . " ";

The strlen() has to look into the according tree.