Update to Drupal 8.0.5. For more information, see https://www.drupal.org/node/2679347

core/modules/forum/config/install/comment.type.comment_forum.yml

@@ -1,6 +1,9 @@
 langcode: en
 status: true
-dependencies: {  }
+dependencies:
+  enforced:
+    module:
+      - forum
 id: comment_forum
 label: Comment_forum
 target_entity_type_id: node

core/modules/forum/src/Tests/ForumUninstallTest.php

@@ -119,6 +119,10 @@ class ForumUninstallTest extends WebTestBase {
     $this->drupalPostForm(NULL, array(), t('Delete'));
     $this->assertResponse(200);
     $this->assertFalse((bool) NodeType::load('forum'), 'Node type with machine forum deleted.');
+
+    // Double check everything by reinstalling the forum module again.
+    $this->drupalPostForm('admin/modules', ['modules[Core][forum][enable]' => 1], 'Install');
+    $this->assertText('Module Forum has been enabled.');
   }
 
   /**