add participant data sanitizing

mne_bids/write.py

@@ -511,6 +511,27 @@ def _participants_tsv(raw, subject_id, fname, overwrite=False):
         }
     )
 
+    # Make sure that all entries to data are lists that
+    # contain scalars (i.e. not further lists). Fix if possible
+    for key in data.keys():
+        cur_value = data[key]
+        if not isinstance(cur_value, list):
+            cur_value = [cur_value]
+
+        # Check if all values are scalars
+        new_value = []
+        for cur_item in cur_value:
+            if isinstance(cur_item, list | tuple | np.ndarray):
+                if len(cur_item) == 1:
+                    new_value.append(cur_item[0])
+                else:
+                    raise ValueError(f"Value for key {key} is a list with more "
+                                     f"than one element. This is not supported.")
+            else:
+                new_value.append(cur_item)
+
+        data[key] = new_value
+
     if os.path.exists(fname):
         orig_data = _from_tsv(fname)
         # whether the new data exists identically in the previous data