Creating a new tabular and saving the LyX file gives \begin_inset Tabular <lyxtabular version="3" rows="5" columns="5"> <features> <column alignment="center" valignment="top" leftline="true" width="0(null)"> <column alignment="center" valignment="top" leftline="true" width="0(null)"> ...
The 0(null) should be changed to 0pt.
