Returns the filter expression of a specific column. If no filter is set, a null is returned.
- columnIndex - index to identify the column, where 0 is the first column*, 1 is the second column, etc.
*Column 0 refers to the first column as established during design-time (in Visual Designer). If movable columns is true, then the user could change the column order. removeColumn(0) removes what was originally the first column, even if the user has moved that column.