Fatal error in 3.1.8

Is there anything else u can provide to help me reproduce? Drupal version, anything particular about the setup with the form? Layout builder or paragraphs in the mix?

I figured out the problem. I was extending CustomFlexWidget to create a custom widget, and I had extended the create() function. I was setting $instance->customFieldManager in 3.1.7, I needed to change this to $instance->customFieldTypeManager in 3.1.8, as it appears the property name changed between versions.

A way to prevent this type of issue in the future, would be to remove the call to create() altogether, and then add functions to retrieve the managers that initialize them before returning them. So you can add these getters:

<?php
public function getCustomFieldTypeManager() {
if (!isset($this->customFieldTypeManager)) {
$this->customFieldTypeManager = \Drupal::service('plugin.manager.custom_field_type');
}

return $this->customFieldTypeManager;
}

public function getCustomFieldWidgetManager() {
if (!isset($this->customFieldWidgetManager)) {
$this->customFieldWidgetManager = \Drupal::service('plugin.manager.custom_field_widget');
}

return $this->customFieldWidgetManager;
}

Then change instances of $this->customFieldTypeManager to $this->getCustomFieldTypeManager() and $this->customFieldWidgetManager to $this->getCustomFieldWidgetManager(). This way, if you change the property names in the future, classes that extend your class will not be affected.