Reproduction scenario:

1. Instantiate the Test Management service in a tracker
2. Edit the campaign tracker to rename/delete the "label" field
3. Access the Test Management service

Trace:

PHP Fatal error:  Uncaught TypeError: Tuleap\TestManagement\REST\v1\CampaignRepresentation::getFieldValue(): Argument #2 ($field) must be of type Tracker_FormElement_Field, null given, called in /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/CampaignRepresentation.class.php on line 131 and defined in /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/CampaignRepresentation.class.php:168
Stack trace:
#0 /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/CampaignRepresentation.class.php(131): Tuleap\TestManagement\REST\v1\CampaignRepresentation::getFieldValue()
#1 /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/CampaignRepresentationBuilder.php(83): Tuleap\TestManagement\REST\v1\CampaignRepresentation::build()
#2 /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/CampaignRepresentationBuilder.php(120): Tuleap\TestManagement\REST\v1\CampaignRepresentationBuilder->getCampaignRep