This issue has been made more visible since story #11232: the default template does not include an assigned to field anymore.

Trace:
Catchable fatal error: Argument 1 passed to Tracker_Artifact::getValue() must be an instance of Tracker_FormElement_Field, null given, called in /usr/share/tuleap/plugins/testmanagement/include/TestManagement/REST/v1/AssignedToRepresentationBuilder.php on line 75 and defined in /usr/share/tuleap/plugins/tracker/include/Tracker/Artifact/Tracker_Artifact.class.php on line 1395
Call Stack
# Time Memory Function Location
1 0.0000 242344 {main}( ) ../index.php:0
2 0.3445 11036256 Luracast\Restler\Restler->handle( ) ../index.php:116
3 0.3474 11113272 Luracast\Restler\Restler->call( ) ../Restler.php:304
4 0.3546 11503120 invokeArgs ( ) ../Restler.php:1059
5 0.3546 11503400 Tuleap\TestManagement\REST\v1\CampaignsResource->getExecutions( ) ../Restler.php:1059
6 0.3816 12704360 Tuleap\TestManagement\REST\v1\ExecutionRepresentationBuilder->getPaginatedExecutionsRepresentationsForCampaign( ) ../CampaignsResource.class.php:325
7 0.3828 12723800 Tuleap\TestManagement\REST\v1\ExecutionRepresentationBuilder->getListOfRepresentations( ) ../ExecutionRepresentationBuilder.php:100
8 0.3828 12724528 Tuleap\TestManagement\REST\v1\ExecutionRepresentationBuilder->getExecutionRepresentation( ) ../ExecutionRepresentationBuilder.php:134
9 0.4040 13453552 Tuleap\TestManagement\REST\v1\AssignedToRepresentationBuilder->getAssignedToRepresentationForExecution( ) ../ExecutionRepresentationBuilder.php:120
10 0.4040 13453664 Tuleap\TestManagement\REST\v1\AssignedToRepresentationBuilder->getExecutionAssignedTo( ) ../AssignedToRepresentationBuilder.php:51
11 0.4045 13453848 Tracker_Artifact->getValue( ) ../AssignedToRepresentationBuilder.php:75