If you have an add-on that creates custom node types and then uninstall that add-on, assuming the add-on doesn't remove all nodes with that custom type you'll get this error:
[code]
LogicException: Entity (class: ) could not be found in src/XF/Mvc/Entity/Manager.php at line 45
[LIST=1]
[*]XF\Mvc\Entity\Manager->getEntityClassName() in src/XF/Mvc/Entity/Manager.php at line 117
[*]XF\Mvc\Entity\Manager->find() in src/XF/Entity/Node.php at line 120
[*]XF\Entity\Node->getData() in src/XF/Mvc/Entity/Entity.php at line 139
[*]XF\Mvc\Entity\Entity->get() in src/XF/Mvc/Entity/Entity.php at line 92
[*]XF\Mvc\Entity\Entity->__get() in src/XF/Entity/Node.php at line 13
[*]XF\Entity\Node->canView() in src/XF/Mvc/Entity/AbstractCollection.php at line 298
[*]XF\Mvc\Entity\AbstractCollection->XF\Mvc\Entity\{closure}()
[*]array_filter() in src/XF/Mvc/Entity/AbstractCollection.php at line 167
[*]XF\Mvc\Entity\AbstractCollection->filter() in src/XF/Mvc/Entity/AbstractCollection.php at line 299
[*]XF\Mvc\Entity\AbstractCollection->filterViewable() in src/XF/Repository/Node.php at line 136
[*]XF\Repository\Node->filterViewable() in src/XF/Repository/Node.php at line 20
[*]XF\Repository\Node->getNodeList() in src/XF/Pub/Controller/Forum.php at line 50
[*]XF\Pub\Controller\Forum->actionList() in src/XF/Mvc/Dispatcher.php at line 232
[*]XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 85
[*]XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 41
[*]XF\Mvc\Dispatcher->run() in src/XF/App.php at line 1645
[*]XF\App->run() in src/XF.php at line 329
[*]XF::runApp() in index.php at line 13
[/LIST]
[/code]
Now, I realize I could just remove all nodes with that node_type_id but that seems like it could potentially be a bit too destructive if those nodes would contain content, so maybe it'd be a better to have the nodes be ignored if the add-on is uninstalled rather than attempting to load no entity 
