fix: rich progress bar error when resume training (#21000)

(cherry picked from commit 6a5c946)

Author: deependujha

src/lightning/pytorch/CHANGELOG.md

@@ -18,6 +18,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed Allowing trainer to accept CUDAAccelerator instance as accelerator with FSDP strategy ([#20964](https://github.com/Lightning-AI/pytorch-lightning/pull/20964))
 
 
+- fix: rich progress bar error when resume training ([#21000](https://github.com/Lightning-AI/pytorch-lightning/pull/21000))
+
+
 ## [2.5.2] - 2025-06-20
 
 ### Changed

src/lightning/pytorch/callbacks/progress/rich_progress.py

@@ -447,6 +447,11 @@ def _add_task(self, total_batches: Union[int, float], description: str, visible:
             visible=visible,
         )
 
+    def _initialize_train_progress_bar_id(self) -> None:
+        total_batches = self.total_train_batches
+        train_description = self._get_train_description(self.trainer.current_epoch)
+        self.train_progress_bar_id = self._add_task(total_batches, train_description)
+
     def _update(self, progress_bar_id: Optional["TaskID"], current: int, visible: bool = True) -> None:
         if self.progress is not None and self.is_enabled:
             assert progress_bar_id is not None
@@ -531,6 +536,9 @@ def on_train_batch_end(
         batch: Any,
         batch_idx: int,
     ) -> None:
+        if not self.is_disabled and self.train_progress_bar_id is None:
+            # can happen when resuming from a mid-epoch restart
+            self._initialize_train_progress_bar_id()
         self._update(self.train_progress_bar_id, batch_idx + 1)
         self._update_metrics(trainer, pl_module)
         self.refresh()