FlexTable extends HTMLTable. As per javadoc for HTMLTable method clear()
Removes all widgets from this table, but does not remove other HTML or text contents of cells.
If you really need to remove all table rows from it use removeAllRows()
instead. See HTMLTable#clear() and FlexTable#removeAllRows()
Note: for better performance you could also consider using Grid instead if you do not need any particular features of FlexTable like dynamic size or colspan/rowspan.