Removing a column displays a popup saying "undefined"

[DavidLeoni]

I created a column by accident in step 3. When trying to remove it, I get a popup saying "undefined". Afterwards the column gets removed correctly.

Please investigate why the popup appears and how to get rid of it.

[DavidLeoni]

It's actually worse than just the popup, seems it prevents undoing the operation

[DavidLeoni]

We should also define some policy for allowing removing columns. Columns can be of three types:

Type	Mapped	Removable	Editable	Notes
Original	no	no	no	Contains data from original dataset
Target	yes	yes??	no	Currently target columns may contain original data, which is hence lost if they are edited. Also, what about automatically created columns, like when mapping from custom sources? Should we allow deletion of such columns?
Working	no	yes	yes	Created by the user