check `valid_indices` shape

official/vision/beta/projects/panoptic_maskrcnn/modeling/layers/panoptic_segmentation_generator.py

@@ -163,7 +163,13 @@ class PanopticSegmentationGenerator(tf.keras.layers.Layer):
 
     # filter instances with low confidence
     sorted_scores = tf.sort(scores, direction='DESCENDING')
-    loop_end_idx = tf.where(sorted_scores > self._score_threshold)[-1, 0] + 1
+
+    valid_indices = tf.where(sorted_scores > self._score_threshold)
+
+    # if no instance has sufficient confidence score, skip merging
+    # instance segmentation masks
+    if tf.shape(valid_indices)[0] > 0:
+      loop_end_idx = valid_indices[-1, 0] + 1
       loop_end_idx = tf.minimum(
           tf.cast(loop_end_idx, dtype=tf.int32),
           self._max_num_detections)