Fix crash when user clicks on "Task Instance Details" caused by start_date being None

airflow/models/taskinstance.py

@@ -823,7 +823,8 @@ def get_previous_start_date(
         """
         self.log.debug("previous_start_date was called")
         prev_ti = self.get_previous_ti(state=state, session=session)
-        return prev_ti and pendulum.instance(prev_ti.start_date)
+        # prev_ti may not exist and prev_ti.start_date may be None.
+        return prev_ti and prev_ti.start_date and pendulum.instance(prev_ti.start_date)
 
     @property
     def previous_start_date_success(self) -> Optional[pendulum.DateTime]:

tests/models/test_taskinstance.py

@@ -1488,6 +1488,39 @@ def test_previous_start_date_success(self, _, schedule_interval, catchup) -> Non
         assert ti_list[3].get_previous_start_date(state=State.SUCCESS) == ti_list[1].start_date
         assert ti_list[3].get_previous_start_date(state=State.SUCCESS) != ti_list[2].start_date
 
+    def test_get_previous_start_date_none(self):
+        """
+        Test that get_previous_start_date() can handle TaskInstance with no start_date.
+        """
+        with DAG("test_get_previous_start_date_none", start_date=DEFAULT_DATE, schedule_interval=None) as dag:
+            task = DummyOperator(task_id="op")
+
+        day_1 = DEFAULT_DATE
+        day_2 = DEFAULT_DATE + datetime.timedelta(days=1)
+
+        # Create a DagRun for day_1 and day_2. Calling ti_2.get_previous_start_date()
+        # should return the start_date of ti_1 (which is None because ti_1 was not run).
+        # It should not raise an error.
+        dagrun_1 = dag.create_dagrun(
+            execution_date=day_1,
+            state=State.RUNNING,
+            run_type=DagRunType.MANUAL,
+        )
+
+        dagrun_2 = dag.create_dagrun(
+            execution_date=day_2,
+            state=State.RUNNING,
+            run_type=DagRunType.MANUAL,
+        )
+
+        ti_1 = dagrun_1.get_task_instance(task.task_id)
+        ti_2 = dagrun_2.get_task_instance(task.task_id)
+        ti_1.task = task
+        ti_2.task = task
+
+        assert ti_2.get_previous_start_date() == ti_1.start_date
+        assert ti_1.start_date is None
+
     def test_pendulum_template_dates(self):
         dag = models.DAG(
             dag_id='test_pendulum_template_dates',