DYN-9845 : handle invalid input (#16728)

Co-authored-by: Bogdan Zavu <[email protected]>

Author: BogdanZavu

src/Libraries/CoreNodeModels/DropDown.cs

@@ -230,7 +230,11 @@ public static int ParseSelectedIndexImpl(string index, IList<DynamoDropDownItem>
             }
             else
             {
-                var tempIndex = Convert.ToInt32(index);
+                if (!int.TryParse(index, out int tempIndex))
+                {
+                    return selectedIndex;
+                }
+
                 selectedIndex = tempIndex > (items.Count - 1) ?
                     -1 :
                     tempIndex;