Removed the same wrong assumption from LotGDTestCase, too.

src/Action.php

@@ -55,9 +55,8 @@ class Action
 
     /**
      * @param string|null $title
-     * @return string|null
      */
-    public function setTitle(?string $title): ?string
+    public function setTitle(?string $title): void
     {
         $this->title = $title;
     }

src/Models/Scene.php

@@ -202,11 +202,17 @@ class Scene implements CreateableInterface, SceneConnectable
     /**
      * Returns a connection group entity associated with this scene by a given name.
      * @param string $name
-     * @return SceneConnectionGroup
+     * @return SceneConnectionGroup|null
      */
-    public function getConnectionGroup(string $name): SceneConnectionGroup
+    public function getConnectionGroup(string $name): ?SceneConnectionGroup
     {
-        return $this->filterConnectionGroupCollectionByName($name)->first();
+        $filtered = $this->filterConnectionGroupCollectionByName($name)->first();
+
+        if (!$filtered) {
+            return null;
+        } else {
+            return $filtered;
+        }
     }
 
     /**

src/PHPUnit/HasAction.php

@@ -78,8 +78,7 @@ class HasAction extends Constraint
                     $checkedOnce = True;
                 }
 
-                # Using KNF, !A or B is only false if A is true and B is not.
-                if ($action->$methodToCheck() == $valueToHave and (!is_null($groupTitle) or $group->getTitle() === $groupTitle)) {
+                if ($action->$methodToCheck() == $valueToHave and (is_null($groupTitle) or $group->getTitle() === $groupTitle)) {
                     $found = $action;
                 }
             }

src/PHPUnit/LotGDTestCase.php

@@ -156,7 +156,7 @@ class LotGDTestCase extends TestCase
                 }
 
                 # Using KNF, !A or B is only false if A is true and B is not.
-                if ($action->$methodToCheck() == $valueToHave and (!is_null($groupTitle) or $group->getTitle() === $groupTitle)) {
+                if ($action->$methodToCheck() == $valueToHave and (is_null($groupTitle) or $group->getTitle() === $groupTitle)) {
                     $found = $action;
                 }
             }

tests/Models/SceneModelTest.php

@@ -364,4 +364,21 @@ class SceneModelTest extends CoreModelTestCase
         $this->expectException(ArgumentException::class);
         $scene1->getConnectionGroup("lotgd/tests/village/hidden")->connect($scene2->getConnectionGroup("lotgd/tests/forest/category"));
     }
+
+    /**
+     * @see https://github.com/lotgd/core/issues/150
+     */
+    public function testIfGetConnectionGroupReturnsNullAndDoesNotThrowATypeError()
+    {
+        $scene1 = $this->getEntityManager()->getRepository(Scene::class)->find("30000000-0000-0000-0000-000000000001");
+
+        # Positively assert that this usually works
+        $connectionGroup = $scene1->getConnectionGroup("lotgd/tests/village/empty");
+        $this->assertNotNull($connectionGroup);
+        $this->assertInstanceOf(SceneConnectionGroup::class, $connectionGroup);
+
+        # Assert that connectionGroup is null if it does not exist.
+        $connectionGroup = $scene1->getConnectionGroup("this-does-not-exist");
+        $this->assertNull($connectionGroup);
+    }
 }