update find unused wells

DB_utls.py

@@ -73,8 +73,10 @@ def find_unused_wells():
         row = well[0]  # A, B, C, ...
         col = int(well[1:])  # 1, 2, ..., 12
         return (row, col)
-
-    empty_wells = sorted(df["well"].tolist(), key=well_sort_key)
+    if "well" in df.columns:
+        empty_wells = sorted(df["well"].tolist(), key=well_sort_key)
+    else:
+        empty_wells = []
     #print(empty_wells)
     
     # close connection