Some artifact link types can go missing when a plugin is desactivated/uninstalled. If this type of links was used in a artifact, the artifact link fields will not be too happy about it and it will throw notices.

Reproduction scenario:
1. Use a type of link from a plugin in a artifact (e.g. _covered_by)
2. Go to the site administration and disable the plugin providing the type
3. Go back to the artifact


Trace:
Notice: Trying to get property 'forward_label' of non-object in /usr/share/tuleap/plugins/tracker/include/Tracker/Report/Tracker_Report_Renderer_Table.class.php on line 1212
Call Stack
# Time Memory Function Location
1 0.0002 407760 {main}( ) .../index.php:0
2 0.0072 2624624 Tuleap\Request\FrontRouter->route( ) .../index.php:49
3 0.0088 2853528 Tuleap\Request\FrontRouter->routeHandler( ) .../FrontRouter.php:110
4 0.0208 4108904 Tuleap\Tracker\TrackerPluginDefaultController->process( ) .../FrontRouter.php:250
5 0.0208 4108936 TrackerManager->process( ) .../TrackerPluginDefaultController.php:45
6 0.0208 4108976 TrackerManager->processSubElement( ) .../TrackerManager.class.php:160
7 0.0268 4569432 Tracker_FormElement_Field_ArtifactLink->process( ) .../TrackerManager.class.php:142
8 0.0443 5180160 Tracker_FormElement_Field_ArtifactLink->fetchRendererAsArtifactLink( ) .../Tracker_FormElement_Field_ArtifactLink.class.php:887
9 0.0443 5180160 Tracker_Report_Renderer_Table->fetchAsArtifactLink( ) .../Tracker_FormElement_Field_ArtifactLink.class.php:668
10 0.0552 5721048 Tracker_Report_Renderer_Table->fetchTBody( ) .../Tracker_Report_Renderer_Table.class.php:497
Notice: Trying to get property 'shortname' of non-object in /usr/share/tuleap/plugins/tracker/include/Tracker/Report/Tracker_Report_Renderer_Table.class.php on line 1218
Call Stack
# Time Memory Function Location
1 0.0002 407760 {main}( ) .../index.php:0
2 0.0072 2624624 Tuleap\Request\FrontRouter->route( ) .../index.php:49
3 0.0088 2853528 Tuleap\Request\FrontRouter->routeHandler( ) .../FrontRouter.php:110
4 0.0208 4108904 Tuleap\Tracker\TrackerPluginDefaultController->process( ) .../FrontRouter.php:250
5 0.0208 4108936 TrackerManager->process( ) .../TrackerPluginDefaultController.php:45
6 0.0208 4108976 TrackerManager->processSubElement( ) .../TrackerManager.class.php:160
7 0.0268 4569432 Tracker_FormElement_Field_ArtifactLink->process( ) .../TrackerManager.class.php:142
8 0.0443 5180160 Tracker_FormElement_Field_ArtifactLink->fetchRendererAsArtifactLink( ) .../Tracker_FormElement_Field_ArtifactLink.class.php:887
9 0.0443 5180160 Tracker_Report_Renderer_Table->fetchAsArtifactLink( ) .../Tracker_FormElement_Field_ArtifactLink.class.php:668
10 0.0552 5721048 Tracker_Report_Renderer_Table->fetchTBody( ) .../Tracker_Report_Renderer_Table.class.php:497