Use script.php to delete the obsolete tasks

administrator/components/com_admin/script.php

@@ -230,7 +230,7 @@ protected function uninstallExtensions()
              * 'pre_function' => Name of an optional migration function to be called before
              *                   uninstalling, `null` if not used.
              */
-             ['type' => 'plugin', 'element' => 'demotasks', 'folder' => 'task', 'client_id' => 0, 'pre_function' => null],
+             ['type' => 'plugin', 'element' => 'demotasks', 'folder' => 'task', 'client_id' => 0, 'pre_function' => 'deleteDemoTasks'],
         ];
 
         $db = Factory::getDbo();
@@ -284,6 +284,31 @@ protected function uninstallExtensions()
         }
     }
 
+    /**
+     * This method deletes demo tasks of the obsolete demo task plugin.
+     *
+     * @param   \stdClass  $data  Object with `extension_id`, `enabled` and `params` of the extension
+     *
+     * @return  void
+     *
+     * @since   __DEPLOY_VERSION__
+     */
+    private function deleteDemoTasks($data)
+    {
+        $db = Factory::getDbo();
+
+        try {
+            $db->setQuery(
+                $db->getQuery(true)
+                    ->delete($db->quoteName('#__scheduler_tasks'))
+                    ->where($db->quoteName('type') . ' = ' . $db->quote('demoTask_r1.sleep'))
+            )->execute();
+        } catch (\Exception $e) {
+            echo Text::sprintf('JLIB_DATABASE_ERROR_FUNCTION_FAILED', $e->getCode(), $e->getMessage()) . '<br>';
+            throw $e;
+        }
+    }
+
     /**
      * Update the manifest caches
      *