DropdownList Multiple is not holding the selected values

I am working with Nested Content, as per my requirement I need to have a dropdown list multiple , based on these values I have to populate the data. But for some reason, my dropdown list multiple in Nested Content list is not holding the selected values.

In  backend , when I select values in dropdown list and save , later when I navigate to the list it does not show any selected values.

Thus, while rendering the dropdownlist seleted values, its getting null exception. Could you please suggest me with regards to this issue.


This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/63882-dropdownlist-multiple-is-not-holding-the-selected-values