Fix bug in the implementation for example_dag4

dagfactory/dagbuilder.py

@@ -852,19 +852,23 @@ def topological_sort_tasks(tasks_configs: dict[str, Any]) -> list[tuple(str, Any
         (https://en.wikipedia.org/wiki/Topological_sorting#Kahn's_algorithm)
 
         The complexity is O(N + D) where N: total tasks and D: number of dependencies.
+
+        :returns: topologically sorted list containing tuples (task name, task config)
         """
         # Step 1: Build the downstream (adjacency) tasks list and the upstream dependencies (in-degree) count
         downstream_tasks = {}
         upstream_dependencies_count = {}
 
-        for task_id, _ in tasks_configs.items():
-            downstream_tasks[task_id] = []
-            upstream_dependencies_count[task_id] = 0
+        for task_name, _ in tasks_configs.items():
+            downstream_tasks[task_name] = []
+            upstream_dependencies_count[task_name] = 0
 
-        for task_id, task_conf in tasks_configs.items():
+        for task_name, task_conf in tasks_configs.items():
             for upstream_task in task_conf.get("dependencies", []):
-                downstream_tasks[upstream_task].append(task_id)
-                upstream_dependencies_count[task_id] += 1
+                # there are cases when dependencies contains references to TaskGroups and not Tasks - we skip those
+                if upstream_task in tasks_configs:
+                    downstream_tasks[upstream_task].append(task_name)
+                    upstream_dependencies_count[task_name] += 1
 
         # Step 2: Find all tasks with no dependencies
         tasks_without_dependencies = [