Flutter: The argument type 'LayoutGrid' can't be assigned to the parameter type 'Widget?'.dartargument_type_not_assignable

How to assign LayoutGrid which is from the flutter package Flutter_layout_grid to Widget?

I think this error is due to null safety and cant figure out how to make conversions for that.

Widget? layout = LayoutGrid(
        columnGap: 12,
        rowGap: 12,
        areas: '''
          r R B B  B
          r R Y Y  Y
          y R Y Y  Y
          y R g b yy
        ''',
        // A number of extension methods are provided for concise track sizing
        columnSizes: [1.0.fr, 3.5.fr, 1.3.fr, 1.3.fr, 1.3.fr],
        rowSizes: [
          1.0.fr,
          0.3.fr,
          1.5.fr,
          1.2.fr,
        ],
        children: [
          // Column 1
          gridArea('r').containing(Container(color: cellRed)),
          gridArea('y').containing(Container(color: cellMustard)),
          // Column 2
          gridArea('R').containing(Container(color: cellRed)),
          // Column 3
          gridArea('B').containing(Container(color: cellBlue)),
          gridArea('Y').containing(Container(color: cellMustard)),
          gridArea('g').containing(Container(color: cellGrey)),
          // Column 4
          gridArea('b').containing(Container(color: cellBlue)),
          // Column 5
          gridArea('yy').containing(Container(color: cellMustard)),
        ],
      ),